How to Easily Fix a Failed Smog Test Vehicle

smog test shop fail

Smog test time? When a car fails smog it means the emissions analyzed from the exhaust tail pipe is too high, there are missing emission components, or you have a check engine light on. We have listed the most common smog related problems and how to fix them quickly.


Before heading into the smog shop it's best to drive the vehicle at least 3 miles to completely warm up the engine and catalyst material inside the catalytic converter which is located in the exhaust system. This additional heat helps eliminate hydrocarbons which produce air pollution and will help your car pass your smog test with flying colors.

What to Look For?

Before choosing a smog shop, check the Google and Yelp ratings, there are a few ways to manipulate this test and ratings are a good way to detect customer frustration and fraud. We also like to see shops that offer a free retest, this helps you save money if your car has a slight issue and the shop is kind enough to help re-test it.

Instant Fail

Problem 1 - If your car's check engine light is on the car will not pass smog, but fear not, most of the time a check engine or service engine soon light can be fixed quickly by replacing an oxygen sensor or getting a new gas cap. To be sure of the problem simply connect a code reader to retrieve the trouble code which will be accompanied by the code identification.

Problem 2 -  If you can feel an engine misfire when accelerating your car there will be excessive emissions, the smog test will pick up on this and it will hinder the test. Replacing the spark plugs or giving the engine a tune up can help repair the problem and pass the test.

Problem 3 - While sitting at a stop light does your car stall out? This is a sign of a vacuum leak or a dirty throttle bore, either problem can be fixed with everyday tools using a new piece of rubber vacuum line or using throttle body cleaner and a shop towel to clean the throttle actuator (bore).

Emission Fails

This section can help point you in the right direction if your car is running good and with no check engine light, but the carbon monoxide (CO2) or NOx levels are too high.

Problem 1 - Your car's engine has an air filter which can become plugged due to severe conditions or neglect causing high emission output, this filter should be replaced regularly. Fortunately this is a very easy job to do for most people.

Problem 2 - A weak or worn out catalytic converter will allow the emissions to be over the legal limit causing a smog test fail. To confirm this is the problem a simple catalytic converter performance test will let you know if the material inside the cat has broken apart or is weak which warrants replacement.

Problem 3 - An EGR valve which is located on the engine is used to lower NOx (nitrogen oxide) which is made inside the engine's combustion chamber if temperatures exceed 2500 F. This valve can be vacuum or electrically operated and can be easily checked using this EGR test video. If you need the location of the valve please ask one of our experts, it's free.

Problem 4 - The air metering sensor used to provide engine air intake feedback information can become "off" with age and may not produce a check engine light code. This Mass Air Flow (MAF) or Manifold Absolute Pressure (MAP) sensor will need to be cleaned or replaced to correct the problem.


Our certified technicians are ready to answer smog test fail questions for free. We hope you saved money and learned from this guide. We are creating a full set of car repair guides. Please subscribe to our 2CarPros YouTube channel and check back often for new videos which are uploaded regularly.

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