Smog Test Failure Repair

Easy step by step instructions on how to correct a vehicle that has failed smog, though appearances may vary, the process is similar for most vehicles.

Difficulty Scale: 4 of 10

Step 1 - One of the most common causes of a failed smog test is the check engine light is "ON" while the engine is running. Learn more

Check Engine Light

Step 2 - The catalytic converter is a vital part of the emission control system, when this item starts to fail it will allow high emissions. (Note: A new cat can overcome a worn or mal-performing engine.) Learn more

Catalytic Converter Replacement

Step 3 - A misfiring engine causes excessive emissions to be produced, correcting this problem will help pass a smog inspection. Learn more

Fouled Spark Plug

Step 4 - Engine tune up's are needed to keep the engine running efficiently and is a required service on all internal combustion vehicles. Learn more

Engine Tune Up

Step 5 - A mass air flow sensor or MAF measures air intake volume and supplies feedback data for the computer which then adjusts the amount of fuel consumed by the engine. When this sensor becomes contaminated it can alter the data produced by the sensor. Learn more

Mass Air Flow Sensor - MAF

Step 6 - An oxygen sensor is designed to measure the amount of oxygen present in the exhaust system while the engine is running. Learn more

Oxygen Sensor

Step 7 - An intake vacuum leak can cause an engine to fail the state smog inspection due to the unmetered air allowed to enter the engine. Learn more

Engine Intake Vacuum Leak

Step 8 - Low cylinder compression can cause an engine to produce excessive emissions which can be tested using a compression gauge. Learn more

Check Engine Compression

Step 9 - On older engine's an EGR valve is used to lower the production of Nox gases which are harmful to humans, if this valve stops functioning a smog test will fail. Learn more

Helpful Information

When a smog test is due its best to take the vehicle to the smog station after its been driven for a time to effectively warm the engine and exhaust system. This helps the engine completely burn it's fuel while the catalyst inside the catalytic converter works at its maximum potential and minimize emissions.


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


Please use our question form if you have a specific question about your car as we are not able to give you a full answer on this page.

Article first published