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Preparing for a Smog Test: Top Five Myths/Tips

Updated on October 03, 2016
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The Dreaded California Smog Inspection

In California the law requires you to smog check your vehicle once every two years, or if your vehicle is tagged as a high-polluting vehicle, once a year. The smog check is mandatory; if you don't do it, you can't complete your next year’s registration. This in turn means that you are not allowed to legally drive your car in California. I have experience working at a Smog Check station in Sacramento, and I hope to shed light on a few tricks people recommend that will supposedly help you pass the smog check with flying colors. Hopefully you will learn something new, and avoid getting scammed buying products or parts you don't need.

Myth No. 1: You Should Warm Up Your Car at High RPMs Before Getting it Smogged

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Should you warm up your car at high RPMs before getting it smogged? The answer is yes and no.

Let's start with the "yes" part. Warming up your car does get your motor running in its best overall condition and allows your catalytic converter to work at the proper level. So yes, having a warmed-up vehicle does help to pass the smog check.

Now, the "no" part of the answer. As a practical matter, it doesn't do any good to warm up your car before a smog test. Supposing you do follow this suggestion, and sit in your car for 5 minutes at 3000 RPM before handing the keys to the smog tech. Then, more than likely, at least ten minutes will pass before the check begins, and in those ten minutes your catalytic converter has cooled off and the effect of anything "extra" you have done to warm up your car is gone. You have only wasted time and gas, and looked like a fool sitting in your car warming it up.

Myth No. 2: Fuel Cleaners and Octane Boosters Help You Pass the Test

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Do fuel cleaners and octane boosters help? Again, the answer is yes and no.

Fuel cleaners will help your car’s engine get clean. This means that a mass of gunk stuck to your engine's valves and head is going to get removed. To get it removed it must be burned, and to be burned it goes into your cylinder head and releases different types of emissions. It's a good idea to use a fuel cleaner often; but it's a bad idea to put it in the car for the first time right before you are taking your car to get smogged.

Octane boosters are another suggestion that won't help you pass a smog test. They enrich the fuel of your car, meaning the engine will burn the fuel, any additives, and anything else at hand. Anything burning inside the motor leaves via the tail pipe, and any new stuff in the mix might cause you to fail your tail pipe emissions test. So don't use these additives right before a smog test.

Myth No. 3: You Need a Tune-Up to Pass

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Actually, more often than not, this myth is true. If you car is not running right, or is misfiring, it will most likely fail the smog test. If you have a bad misfire, this could cause your check engine light to come on also, meaning you will fail the functional portion of the smog inspection.

Getting a tune-up is usually a plus in more than one way. Not only will it increase your chance of passing a smog test, your vehicle will run a lot smoother and you'll get more miles per gallon. Simple changes to the operation of your spark plugs, wires, cap, and rotor, and replacing the air filter and fuel filter when needed, can really make a big difference. Follow the manufacturer's recommendations on when to maintain your vehicle and keep your car running in prime condition.

Myth No. 4: Your Smog Guy Told You To Just Replace the Cat

Catalytic Converter or "Cat"
Catalytic Converter or "Cat" | Source

Many smog check places make this suggestion, but replacing your catalytic converter is usually one of the last things that you should mess with. Get advice and find out exactly what the problem is before okaying the replacement of an expensive part. Your smog technician may be recommending a cat just to make a quick fix so that your car will pass this week. But you may find in succeeding years that each time you take your vehicle for a smog inspection, it will either need a catalytic converter to pass, or it will fail for sure. I suggest that you take your vehicle to a licensed smog repair shop, and have them diagnose the issue correctly. If your catalytic converter is going bad, something is causing it to go bad. Rather than just replace your catalytic converter each time, identify and fix the problem.

Myth 5: You Should Pull Out Your "Check Engine" Light Before the Smog Check

"Check Engine" light (yellow motor icon) on dashboard
"Check Engine" light (yellow motor icon) on dashboard | Source

This is one of the most common things people try to do to get their car to pass, and one of the silliest. Most vehicles from 1990 and later are equipped with a check engine light. This light is required to be working when the technician tests the vehicle. If the tech doesn't see the light, he must check his book to see if the vehicle comes with a light. If you vehicle comes equipped with a check engine light, but it does not work—either because it went out or because someone tampered with it—your car will automatically fail. Make sure your check engine light works. On GM cars, the light is a little legend, "Service engine soon", and on most other makes it's a little yellow engine-shaped light. If your check engine light stays on, it means that for some reason your engine is not running in the cleanest and most economical way, and so you may as well have your car serviced.

The Grand Finale

I hope you learned a few ways to make sure you do not fail your smog inspection. Always follow the manufacturer's guidelines for service intervals. Make sure that you don't do something that could ruin your vehicle's smog history, since it stays with your car forever.

California Smog Certificate

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    • profile image

      Amanda 4 years ago

      i drive a 1995 toyota corolla that runs great except that about a month ago the trip meter went out and then the fuel gauge started to act up too. I have gas but it flexuates weird and I have no money to get it looked at by a mechanic and everyone at autozone has no idea what it could be and refer me to a mechanic. All other electrical functions perfectly. Will the problems I am experencing effect my smog outcome? Thanks in advance.

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      Wang 2 years ago

      I find it interesting that both mars that i own are required to go to STAR STATION. neither one fails the test. here are the results of the 1987 BMW 528e odo reads 270k miles on ODO.

      Test results: 15mph HC max allow 80 meas 26 CO max allow 0.52 meas 0.24 NO max allow 785 meas 511.

      25mph HC max allow 62 meas 20 CO max allow 0.42 meas 0.14 NO max allow 702 meas 437

      2003 VW Golf 2.0l 117k on the ODO

      Test results: 15mph HC max allow 58 meas 17 CO max allow 0.52 meas 0.04 NO max allow 451 meas 172.

      25mph HC max allow 42 meas 14 CO max allow 0.50 meas 0.05 NO max allow 738 meas 175.

      again why are both cars required to go to STAR STATION?

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      mcm 2 years ago

      You car is old.

      "The left" wants old cars off the road and you driving a new Prius.

      My 1994 NSX with 60k has a hard time every 2 yrs. they just want them off the road. Sad.

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      duh 8 months ago

      Total lefty. Old cars. I make them pass emissions by keeping them running right. Even my 76 BMW 2002 has done just fine. 85 Land Cruiser FJ60 has been hardest. Right wingers caused that though through emissions control requirements during the fuel shortage in the 80s. Keep wearing your tin foil hat, mcm.

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      GrizzlyBear 7 months ago

      Myth No. 1: You Should Warm Up Your Car at High RPMs Before Getting it Smogged is contradicting.

      The "yes" part states, "Warming up your car does get your motor running in its best overall condition and allows your catalytic converter to work at the proper level. So yes, having a warmed-up vehicle does help to pass the smog check." But as the "no" part states, "Then, more than likely, at least ten minutes will pass before the check begins, and in those ten minutes your catalytic converter has cooled off and the effect of anything "extra" you have done to warm up your car is gone."

      Likewise, the "no" part states, "As a practical matter, it doesn't do any good to warm up your car before a smog test." What a minute! But the "yes" part stated, "Warming up your car does get your motor running in its best overall condition and allows your catalytic converter to work at the proper level. So yes, having a warmed-up vehicle does help to pass the smog check."

      So which is it? If "yes" then do not contradict your reason for "yes" in the "no" part and vice versa.

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      Brian 2 months ago

      Would I have to take out the turbo out of my car inorder to pass smog?

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