Updated date:

Tips and Advice on How to Pass a Smog Test

No matter what you do, some cars just won't pass the smog test.

No matter what you do, some cars just won't pass the smog test.

Knowing how to pass a smog test is one of those essential bits of knowledge that every car owner in California should know. But there are a lot of pitfalls involved in passing a smog test. Some people inadvertently shoot themselves in the foot doing things that they'll think should help them pass a smog test but, in fact, cause them to fail.

A Few Misconceptions About Smog Checks

  1. There's nothing I can do to help my car pass. Wrong. There's a lot you can do, which we'll talk about in the next section.
  2. Higher octane fuel will help me pass. The biggest mistake people make is buying more expensive fuel. People often have the misconception that how to pass a smog test is to use premium 91 octane fuel. It seems to make sense, right? More expensive fuel is probably purer, which means it probably burns cleaner and gives off fewer emissions right? Wrong. 91 Octane fuel contains higher amounts of combustible chemicals, which means it's formulated to burn longer, in higher performance engines. If you put it in an engine that's not meant for high performance, the fuel won't completely burn up inside the chamber, meaning that every time a piston fires, a little bit of unburnt fuel will be pushed out of the exhaust, causing much MORE emissions to come out of the engine.

Tips on How to Pass a Smog Test

These tips will not only help vehicles pass a smog test, but run at higher performance and get better gas mileage:

  1. Use low octane fuel. The lower the octane the better. We already discussed above that buying premium fuel can cause you to utterly fail your smog test. Go the other way. If you can find 85 octane fuel, use it. Don't use it permanently—just use it to pass the smog test. What happens is that lower octane fuels burn up faster while still in the combustion chamber, giving your engine the maximum amount of time to incinerate those pesky emissions, before sending them on their way.
  2. Do a tuneup. Cars run inefficiently after awhile because the individual components are either clogged or wearing out. If you clean or change your air filter, get new spark plugs, and get an oil change, you'll notice your car is running at higher peak efficiency.
  3. Use a heavier oil. Vehicles that burn oil have more emissions. Old vehicles are especially notorious for leaking a bit of oil into the combustion chamber. That's why heavier oils were invented, to slow down these little leaks. If you know your engine is burning a little bit of oil, ditch the 5w30 or 10w30 and ask them to put in something like 10w40 or thicker.
  4. Use a fuel injection cleaner a few weeks beforehand -- It's always a good idea for your engine and fuel system to be as clean as possible before you go into a smog station. A few weeks before your trip, pour a bottle of fuel injection cleaner into your gas tank when you go for a fill up at the gas station. This will hopefully take care of some of the gunk and grime clinging to your fuel system.
  5. Use a fuel treatment system. If you're really worried about passing your smog test, buy a fuel treatment system that specializes in lowering emissions. Lucas Fuel Treatment is one such specialized formula that seems to get some great results. I've read a lot of testimonials online and people seem to really like it.
  6. Clean your MAF sensor. The mass air flow sensor (MAF) helps to determine the amount of air to add to the fuel/air mixture. If your MAF sensor is dirty, it can cause the fuel mixture to be too rich in gasoline, which can lead to excess emissions.

Reject This "Tip" on Rubbing Alcohol

While using fuel additives like methanol and ethanol to pass a smog test is illegal, some say that rubbing alcohol (isopropyl alcohol) might be legal, though I wasn't able to find anything definitive on that. In general, adding alcohol to the fuel is risky. It can damage rubber seals that are very difficult to replace, once damaged. Damaged rubber seals within the motor can cause oil leaks into the combustion chamber and other unpleasantness. To be safe—and to make sure you're not breaking the law—I'd say pass on the rubbing alcohol.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


Jeff on April 09, 2020:

I’ve used methanol several times in the past it will work but for vehicles 95 and older. They’re right and In less you have a lot of experience with it you can do more damage than good.

Mehrdad on October 27, 2018:

Thanks for the information.

Kami on April 19, 2017:

Thanks for the information. Unfortunately I read it after I put the fuel injection cleaner in my gas tank filled it up with premium gas and then took it in for the smog check for it fail.

Luckily the mechanic told me I should burn off all this gas and then bring it back.

I came back and started researching how to pass a smog test and found this information on all the things to not do that I did. SMH

chris on November 29, 2014:

My mistakes lol sorry I had like a third tank regular chevron 87 but then filled up the rest with 89 along with that lucas deep fuel system cleans and I plan to get smog check in two days and will probably have a half of tank left.. I wish i would have read about how low octane is better but too late my tank is full hopefully I should be good I have a 06 lacrosse

chris on November 29, 2014:

I meant 98 not 91

chris on November 29, 2014:

So did I mess up and fill up tank with chevron 91 and put lucas deep fuel system clean and need to get smog check in 2 days

Benji Mester (author) from San Diego, California on May 15, 2012:

Thanks! You're welcome.

reviewthelaw from Newport Beach , California , 92660 United States on May 11, 2012:

Benjimester! you have such informative hub. I'm not familiar with this, however I am greatful you shared it to us.

Benji Mester (author) from San Diego, California on February 12, 2012:

Yeah that's a really good tip. Thanks Ron.

Ron on February 11, 2012:

On additional piece of advice CHANGE YOUR OIL & FILTER! Dirty oil is high in hydrocrabons and all engines will burn claener and produce less emmissions when they have new, clean oil and a clean oil filter.

Benji Mester (author) from San Diego, California on June 07, 2011:

Nice, I haven't heard of Mega Power before. Anything that helps lower emissions is a good thing. I'll have to check that out.

George Christ from http://www.auto tune up and repair options.com on June 06, 2011:

I have added a product called Mega Power FISC to help my older cars trucks and tractors that lower their emissions. This product works by cleaning several areas of the motor that causes higher emissions. And anybody can use this product. While doing so it seems to boost performance significantly. You're right. Do any cleaning a week in advance to let the chemicals do their thing and dissipate.

Benji Mester (author) from San Diego, California on March 30, 2011:

Very cool. I'll check that out. Thanks for the link.

Rob Bell on March 30, 2011:

I got my car through our exhaust emissions test here in the UK by using BG44K Power Enhancer.

It cleans the fuel injectors too.


Benji Mester (author) from San Diego, California on March 24, 2011:

Yeah, I think a lot of people are in that same boat :) California is super strict with emissions laws, but most everyone else is pretty lax. Thanks very much for stopping by.

Matt on March 24, 2011:

Thank goodness my 1988 Camaro doesn't have to do the smog test here in Missouri. Nor are there visual inspections except for the catalytic converter which I keep mine on. My car would never pass if I had to!

Benji Mester (author) from San Diego, California on December 23, 2010:


Rismayanti from Tropical Island on December 23, 2010:

Good tips.. thanks for share

Benji Mester (author) from San Diego, California on November 10, 2010:

I made some changes to be on the safe side, since I didn't find a definitive answer. I was never really sold on the isopropyl alcohol idea in the first place, so the changes are a good thing. Thanks again for the link.

Benji Mester (author) from San Diego, California on November 10, 2010:

I've never heard that before but I'll have to check it out. Thanks for the link.

OhMan on November 10, 2010:

Is using "Isopropyl Alcohol" considered illegal? See http://www.smogtips.com/smogcheckfraud.cfm

Benji Mester (author) from San Diego, California on May 18, 2010:

Thanks Alek! It's very good to see you. I hope all is well.

Nancy Hinchliff from Essex Junction, Vermont on May 18, 2010:

Well, guys, I don't live in California but, if I did, I would certainly be beholden to you two for giving me some real improtant information. Thanks.

Related Articles