How to Pass a Smog Test
This article is mostly for older cars and limited budgets. The info is generally organized in ascending order of costs involved. There are also lots of maintenance tips.
This article could also be titled, "The Top 10 Ways to Not Flunk the Smog Test" since failure to implement many of the items here would probably cause grief.
Interestingly, in Great Britain the smog test is called the MOT test. Over there, in addition to the smog aspects, they test lights, steering, tires, seat belts, brakes, body structure, driver view and whatever else they can possibly think of. Welcome to our future . . .
Smog test time is as good a time as any to do right by your car. By the time you’re done, you’ll feel like you’ve got a new car again.
This page is particularly geared to the California biannual smog check. If you can pass the California test, you can probably pass any state's test.
It has long been believed if your car takes regular gasoline, you should fill the tank with premium gas prior to the smog test (See update!).
Premium gas will do:
- Absolutely nothing for your mileage.
- Absolutely nothing for engine maintenance or vehicle life expectancy.
However, it has been believed premium would burn a little cleaner and thus result in lower emissions.
There now seems to be some serious dispute about this. In fact, some are recommending the exact opposite, i.e., putting regular gasoline in a car which normally takes premium for when smog testing time comes around.
This page is recommending just putting the grade of gasoline in the car the engine was designed or adjusted for, i.e., whatever the car manual and engine specs specify.
However, if one really believes in going outside the specs one way or the other as to premium versus regular, then maybe just opt for the middle grade and call it a day.
Does your state mandate 10% ethanol during certain times of the year? If possible, postpone or accelerate your test time so as to take advantage of this. You want your tank full of 10%-ethanol-gas when you take the smog test.
Like premium gas, ethanol does nothing for your car. But it does burn a little cleaner.
Do not get the idea of adding your own extra ethanol (or whatever). It is illegal to do so and you could get caught, not to mention possibly damaging your engine.
Fuel Cleaners and Other Additives
The good news is fuel additives can indeed clean out the carbon buildup and other gunk in your engine and catalytic converter, thus lowering emissions when testing time rolls around.
The bad news is the way the fuel cleaners work. They remove the carbon buildup and gunk by causing it to be burned away. This burning away results in increased emissions. Thus, adding the fuel additive just before the test can cause you to flunk a smog test you might have otherwise passed.
So adding fuel cleaner on a regular basis is usually a good thing. However, adding fuel cleaner within a couple months of taking the smog test is a bad idea.
Any other additives just prior to taking the test is also a bad idea for the same reasons. When those additives are burned, additional emissions are released, possibly resulting in an unwarranted smog test failure.
Water and Temperature
The higher the humidity, the better (and cleaner) your engine runs. A rainy day would be perfect, except there are reports the dynamometer your car will be placed on sometimes doesn't handle wet tires very well (more about tires later).
Temperature is also a consideration. Your vehicle will do better on the smog test when the outside temperature is 60 degrees, as opposed to when it is 90 degrees.
Doing the smog test first thing in the morning is usually your best bet. Besides, people are generally in a better mood then (more about that later).
Do make sure your engine is fully warmed up before arriving at the test station. A good 20-minute drive is recommended. You also definitely want the catalytic converter at full operational temperature.
Be sure your battery is fully charged before the test.
Most people use their car only for short trips most of the time. This does not provide sufficient time for the battery to fully recharge after starting.
When the battery is not fully charged, a weak ignition spark can occur. A weak ignition spark will result in poorer mileage and much higher smog pollutants.
Many people trickle-charge their battery overnight every once in a while. This can add years to the life of a battery. Beware of overcharging.
Be sure there is sufficient distilled water in the battery to completely cover the plates.
A Sears DieHard battery can last over 10 years when properly cared for. Avoid Walmart batteries like the plague. This is just my opinion based on experience. (Also avoid Firestone shops—my two cents.)
Take That Car for a Major Freeway or Highway Drive!
If your car is mostly used for short trips, that is not good. Over time, carbon and all sorts of other gunk have built up in your engine, on your spark plugs, in your crankcase, and in your catalytic converter. All of these will emit noxious chemicals and cause other problems during a smog test. This scenario has caused many a smog test failure.
Like it or not, you have to burn out and blow out all that junk. In other words, there is a freeway or highway trip in your future; the longer, the better. At minimum, you should take a 2-hour round trip. And rush hour traffic doesn't count; the car needs to be driven at the speed limit.
Hopefully, there is some errand, trip, or short planned vacation which can come into play on this.
As a side note, it is absolutely the best time to change the oil and filter after the car has been driven a couple of hours at high speed. All the gunk on the sides and bottom of your crankcase are now temporarily dissolved in the oil. It is the perfect time to get rid of it.
Car Manuals Make Things Easier
Chilton and Haynes publish manuals for most every year, make, and model of car. It is written for the consumer, so you don’t have to be a mechanic with ten years experience to benefit from them.
The manuals will tell you:
- About all routine maintenance that needs to be done; and when and how to do it.
- Where all the car components are (example: where is the PCV?).
- How to do many common repairs and especially how to avoid the many pitfalls/traps that were designed into the vehicle and are just waiting for you.
These manuals will give you the information you need to do everything described in this article and a whole lot more. The instruction level starts at rank beginner and proceeds to as advanced as you care to go.
These manuals will save you many hours of grief and a lot of money. If you plan to keep your car for any length of time, then these books are a good investment.
Your local auto-parts stores like Kragen, Pep Boys, Autozone, Sears, Walmart, or whoever, has a book section. Drop by. If they don't have your particular manual, they can usually order it for you.
Based on your own personal life experiences, the percentage of mechanics that you believe to be honest/ethical is as follows (pick the number that is closest):
Oil and Filter
If your car is due for an oil and filter change, now is the time. Dirty crankcase oil can indeed cause a smog test failure due to the hydrocarbons (HC) part of the test.
This is one of the few maintenance items that it is best to give to a shop to do. You won’t save money doing it yourself but you will save yourself a lot of mess and aggravation. The going rate is around $30. Whatever you decide to do, make absolutely certain the oil level does not exceed the full-mark.
While changing the oil and filter, they’ll do a lot of the other maintenance stuff for you as well. How are the radiator coolant, transmission fluid, power steering fluid, and brake fluid doing? How about the differential, battery water, and tire pressure?
Important note: Many shyster places will try and tell you they have discovered your car needs several hundred dollars worth of additional work. This is particularly applicable to certain national chains.
Do an advanced Google search on the place you intend to visit. The search query should be: “entity-name”; with any/all of the words: “scam," “ripoff," “rip-off." If there are several pages of complaints, then obviously select someplace else.
Ask people you trust for recommendations.
Side note: The days of changing your oil every 3,000 miles are long gone. Naturally the lube shops will advise you otherwise. What does your manual or Chilton say? No manual? Edmunds will tell you. Don't want to mess with Edmunds? Every 8,000 to 10,000 miles seems to be the consensus these days.
The Air Filter and Another Important Filter
Does your car need a new air filter? Now is the time. Wipe away all the dust in the canister when you replace it. Both inside and outside. A dirty air filter will cause incomplete combustion, which will cause high carbon monoxide (CO) emissions, which will cause a flunked smog test.
You might also notice an oil/dirt/grease soaked side-pad in there. It is the filter where the oil vapors from the valve covers are circulated into the combustion cycle. Remove and clean it with Gumout.
Likewise clean the area where it is located. The gunk has been building up there for years and when it comes to parts-per-million test measurements, it is not doing you any favors. Failure to do this one thing alone could cause a smog test failure. It's an easy, simple thing to do. Don't take the chance.
If your car/make/model has something more complicated than a simple side-pad, hopefully it is something you can clean, as opposed to buying a new one.
And if there is no above-described side-pad in the air canister, check all connecting air hoses. There may very well be an inline filter you need to deal with.
If you clean the inside part of the air filter canister where the gunk has built up, the Gumout will remove the paint. Personally, that didn't bother me much. The area was small and would quickly be protected from rust with a new coating of oil anyway.
A clogged/disconnected/broken PCV causes a lot of flunked smog tests. It is an inexpensive and easy part to clean or replace.
Make sure yours is OK. If the thing doesn't rattle when you shake it, it is clogged. Gumout time again.
A fouled spark plug causes a lot of flunked smog tests. If your vehicle is mostly used for short trips, fouled spark plugs can result. Doing a two-hour round trip at normal freeway speeds for some useful endeavor before the test might clean out the carbon and other junk in the engine and plugs.
Do your spark plugs need to be replaced? A mechanic will charge ~$100 to do that for you, plus the inflated cost of the plugs. Or you can earn yourself $100 (tax free) by doing it yourself.
There are a lot of mistakes you can make when changing plugs. It is imperative to know all the traps before proceeding. If you are not already knowledgeable, a how-to manual, online research, a quick trip to the library, or a knowledgeable friend will save the day.
Depending on how the plugs are positioned, changing them can be a lot of work and frustration. You don’t have to do all of them at once. If so inclined, just do one or two at a time as/when convenient.
Some auto parts stores have tool-loaner programs.
Distributor Cap and Rotor
Does your car have a mechanical distributor and rotor? Is it original equipment? Does the car have over 80,000 miles? If you replace the distributor cap and rotor, the improvement in performance will amaze you.
The distributor cap and rotor are easy and inexpensive parts to replace. However, you need to read the manual first, otherwise traps await.
If not replacing the distributor cap, be sure and check it for cracks and other defects. Cracks in the distributor cap can cause the engine to fail the test.
Distributor or Spark Plug Wires
Are the spark plug wires original equipment and over 80,000 miles old? If so, there may be cross-sparking. That’s not good for the engine or the smog test.
If so inclined, observe the engine running in near-total darkness (flashlight anyone?). Look for sparks between the wires or from the wires to metal. Don’t give yourself carbon monoxide poisoning while attempting this. Don’t touch anything. Avoid all moving parts. Leaning over the engine while wearing a tie would be especially unwise.
Better to just go ahead and replace the wires. Mistakes can be made while doing this, so read a manual first. And do just one wire at a time. Attempting a mass disconnect/removal/replace scenario is almost certainly a recipe for grief.
Does your car have a carburetor? Spraying Gumout in it and on it will do wonders; both as to performance and reducing smog pollutants.
Follow the directions on the spray can. Do not overuse. Do not overspray. Do not do just before the smog test.
- Does your check-engine light remain dark when starting the car? If so, you probably have a burned out bulb and you have automatically flunked the smog test.
- Does your check-engine light remain on after the engine has started? The warning light could indicate something minor or something serious. Either way, you have automatically flunked the smog test.
Depending on your make/model/year of car and the nature of the problem, the manuals might be able to solve this for you without having to go to a mechanic. They will tell you how to retrieve the codes without equipment and what those codes mean.
Disconnecting your car battery for a minute could reset your check-engine light back to norm. However, you would lose the stored error codes. Those error codes are what would tell you is wrong. And, of course, those problems will no doubt reoccur the next time you start your car.
Loose or Oxidized Wire Connections
Your car has more computer technology than the 1969 Moon Lander. One poorly connected wire can cause a smog test failure.
Do a visual inspection. Make sure there are no loose wires. Make sure all wires are firmly connected. If you find anything even remotely suspect, disconnect and reconnect the wire a few times to remove oxidation; maybe sand it a little bit with emery paper as needed. If any of the plastic connector/clamp parts are broken; now is the time to take care of them.
Important note: Finding or fixing a wire connection problem could solve a check-engine light issue (will need reset).
Hoses and Vacuum Leaks
Vacuum leaks can cause smog test failures.
Check all those little hoses around the carburetor, air cleaner, PCV, here, there, and everywhere. Make sure all are firmly connected. Make sure none of them have any splits, tears, etc. Replace anything suspect.
Risky business: If you spray a water mist near/around the sides of your carburetor (or fuel injector for that matter) while the engine is running, and the engine speeds up, then you have a vacuum leak.
That is a very probable smog test failure. The risky business part relates as to what to do about it. Can you pinpoint the exact location of the leak? Examine the gasket seams—look for splits/cracks/stains.
If you can determine the exact location of the vacuum leak, you have a decision to make: try to do a kluge fix; give a mechanic some serious money; or do nothing, roll the dice and take the test.
This article will not address as to what that decision should be. Using gasket glop on the side and figuring out when to turn the engine off before it's sucked into the carburetor? You are on your own.
A fixed vacuum leak may solve a check-engine light problem (may need reset).
Engine Timing and Idle Speed
It is common knowledge advancing the timing above specs will increase performance. If the timing is not within hood specifications, you will flunk the smog test.
An out-of-spec engine idle speed will also cause a test failure.
If your vehicle is of the points/dwell generation, then you know the order is:
The previously mentioned resources will give you everything you need to know about replacing the points (if need be), setting or checking and readjusting the dwell, timing and idle.
The factory specs for the engine timing, idle speed, and other info is stamped on a metal plate or decal, usually located towards the front of the engine compartment. One of your local auto parts places may have a tool loaner program for the timing light and the special wrench you may need.
Or you can skip this section and just go ahead and take the smog test. In fact, now would be a good time to mention that the “skip it” option applies to all sections of this article. Only you can decide which items to implement and which to forgo.
Squealing Fan Belt and Seized Smog Pump
Though all the belts are at proper tension and have even been sprayed with dressing, the fan belts are still squealing like a banshee? The cause may very well be a seized smog pump.
Start the engine and look at the pulleys. Are they all spinning smoothly or is one of them actually motionless or moving intermittently? If so, there is your squealing belt. And very likely, that pulley is the one attached to your smog pump. It has frozen. Eventually, it is what every single one of those things always do.
- Finding a replacement at a junk yard. For this option, it would be wise to be very knowledgeable on the subject of junk yards and air pumps. As an example, smog pumps have air filters that must be dealt with. Might as well snag the belt if it's in good shape and cheap.
- Buying a rebuilt one. Side note: they call them "remanufactured".
- Buying an original new one (if any still exist).
- Going to a mechanic.
Personally, I'd rather shoot myself in the foot than do either of the last two options.
First thing to do is learn about smog air pumps in general and what the average prices are online.
Next is a trip to the auto parts store and a long chat with the counter person. Among other things, is an exchange required? What exactly are the prices for new versus rebuilt? Who knows, maybe some entrepreneur has come up with a cheap, new, one-size-fits-all alternative for some engine groups and/or catalytic converter configurations; highly unlikely though. Whatever information you learn, you don't have to make any decisions until you are ready.
You Want to Keep the Pulley from the Old Pump
Depending on how much work room there is in that area of the engine compartment and what other components the fan belt is connected to, you might get lucky and be able to remove the pulley from the pump with the pump still attached to the engine. If so, it is a convenient way to get that done, what with the engine politely holding the pump still for you.
If you do this, leave the old pump in the car until actual replacement time. If the wrong bit of bad luck comes along, you could end up being accused of smog equipment tampering. With the air pump still in the car, you can prove right then and there the thing had frozen and you had no choice. And your fan belt squealing problem is solved. Needless to say, don't forget to remove the belt along with the pulley.
If picking up the replacement pump at a store, bring the pulley. You want to be sure the bolt holes match up.
A side note... If the old unit is not needed for an exchange, I'd be inclined to take it apart just out of curiosity. Maybe some new ideas or options might come to light.
Odds and Ends
Does the vehicle’s exhaust system have any holes in it? If so, you have automatically flunked the smog test.
Sticking a wire brush up the tailpipe and cleaning out the accumulation won't hurt. Do this several days before the test, so the loosened grit has time to be completely blown out.
Does the gas cap not seal properly? If so, you have automatically flunked the smog test.
Safety Test and Tire Pressure
Does your state include a safety test along with the smog test? If not, sooner or later they will get around to that.
- Make sure all your lights work: brakes, turn signals, etc.
- Good a time as any to check the tires for proper pressure, uneven wear, sidewall splits, tread depth, embedded nails, etc. The tire pressure is especially important. The smog test will be performed on a dynamometer. For the car to perform at its best, the tire pressure must be at specifications. Too little or too many pounds-per-square-inch pressure will cause the car to perform poorly in different ways.
A Clean Car Is a Good Karma Car
You want your car to look as good as possible. If it shows you care about your car, maybe the technician will care about you in those borderline situations.
- Wash and clean your car inside and out. This includes removing all biohazards and other trash. Especially check under the seats. Clean the windows inside and out. Wipe/brush the inside dust off everything. A pan of baking soda will remove odors (in a few days).
- Upholstery patch-up kits don’t cost that much.
- Does the body have scratches? Auto stores sell colored “crayons” that will permanently make it look ten times better.
- Come to think of it, when is the last time the poor thing has been waxed?
- Wipe the dust off all the top surfaces in the engine compartment as well.
Presenting the technician with a car that looks like it’s been treated as a PoS is never a good idea. The PoS might translate to a WTF; and your car is smog test history.
As a side note: the same psychology comes into play when you are pulled over for a traffic violation and the officer is considering whether to give you a ticket or not.
Warning! Smog Test Stations Are Run by Humans!
You usually don’t have to worry about the test technician. As an employee, his/her only agenda is to do a quick, fair test and move on to the next one.
You’ve done your part by presenting the technician with a clean, apparently well-maintained car, thus doing nothing which would deter or distract the technician from their original goal.
It is the person at the front counter you have to worry about. If that person instructs the technician to flunk your vehicle, then that is what will probably happen.
There are at least three personality types you need to be aware of:
1. First, of course, is the thief. Depending on how your state sets things up, you may be scammed into paying for unnecessary repairs.
2. Second, is the Self-Appointed-World-Policeman. Some of the things that can "set-off” the self-appointed-world-policeman are: any/all facets of your appearance; the perception you are not a contributing member of society; the perception you are “low-class”; the perception you are “high-class”; the perception you are somehow different in some way, shape, or form from what they consider to be a "normal" human being.
3. Third, is the overly-sensitive, self-defensive personality-type. This personality type usually develops from being hammered in high school for something or the other; usually some sort of physical appearance attribute, though it could be for other reasons as well. Practically anything can set this personality-type off: what you say; what you don’t say; voice intonation; eye-contact, non-eye-contact; use of any word that could be even remotely construed as a double-entendre.
These personality types are more prevalent in some counties/cities than in others. The second and third personality-types may possibly be more prevalent in the smaller shops.
To help reduce your odds of triggering these personality types into “selecting” you, you can do the following:
- Don’t wear anything with writing on it.
- Don’t wear anything with pictures on it.
- Don’t dress rich. Don’t dress poor. Don’t dress sexy.
- Don’t wear any political, religious, or pornographic artifacts.
- If you have tattoos, cover them up if possible.
- If you smoke: do not smoke in the car on the way to the test; do not smoke while waiting for the test.
Of course all of the above is a blatant violation of your civil rights. And is of course completely unfair and outright illegal. However, what do you want to do? Pass the smog test or attempt to “cure” a warped/distorted personality type? It’s up to you.
Here are some other things you can do:
- Do not be adversarial.
- Do not volunteer any unnecessary/irrelevant information.
- Do not engage in any unnecessary/irrelevant conversation whatsoever.
- Be open and honest, yet keep the interaction to a minimum.
- Do not show fear. Do not suck up (sucking up is recognized as fear).
Selecting a Station
Smog shops advertising on the internet are probably your best bet. They know you know how to go to the various forums and trash them if they try anything unethical.
They know you know how to find all the relevant federal, state, county, city, and consumer related addresses if you wish to send complainant emails.
Besides, they are in a very competitive business. Some of them are most likely handing out big, fat coupons or discounts.
Whoever you select, do the same scam search you did for the oil-change shop.
And check the state website to be sure they are really licensed to do smog tests.
If Your Vehicle Flunks
First, it’s not the end of the world. Just a pain in the …
Your state may have an Auto-Repair-Program which will provide financial assistance. Your state may have a Special-Dispensation program to give you time.
The Smog Test Station should have this information. It would also behoove you to troll your state’s DMV website and the secondary/separate smog test website (if they have one). The website(s) might also have a complaint section. The website address(es) should be printed on your Vehicle Registration Renewal Notice.
If you live in California, there is also what is known as a Smog Test Referee Program. If you truly believe your car should have passed the smog test, the smog test referee will do a second smog test to confirm or refute the findings of the first test. There is currently no charge for this second test, subject to change at any time. Other states may or may not have similar programs.
If your car flunks the smog test and you've decided it's just not worth it anymore, some states will pay you to retire your car. You get the cash. They dismantle the car and it goes to scrap.
One way or the other you will get through it; and you will have a nice, “new” car again in the process.
An Addendum—Driving With Expired Tags
This can be a dangerous game. It all depends on which city and/or county you happen to live in.
You put off the test until the last minute; and you flunk it. There is no way the state will complete your financial assistance application in time for you you to get the repairs done before your tags expire.
First, if the state will let you, pay the registration fee before the deadline anyway. You really don't want to be subjected to the no-doubt excessive late payment penalty fees; this is premised on the belief your vehicle will eventually pass the test.
To drive or not to drive—only you can decide. In some places you will only be cited. In other places your vehicle will be impounded. Getting your vehicle out of the impound lot will be a real challenge indeed. Even if you are able to pay the ransom and are allowed to retrieve it, you might be required to pay to have the thing towed on a flatbed truck, presumably to the smog repair shop.
Just being cited may not be much fun either. In many cities and counties, “justice” has been tossed out the window when it comes to traffic tickets. The fine may “only” be $50 to $100, but they are not done yet.
Depending on where you live, there may be an additional $200 to $300 tacked on as “fees." Since they are calling it a “fee”, instead of a fine, they can and do pretty much make up whatever they want.
Bottom line is to do what you need to do for the car and get the test done as soon as possible. By all means use the aforementioned seasonal rainy weather and ethanol stratagems when feasible.
But if you intend to utilize the state's financial aid program, be sure to give yourself sufficient time.