Why the light causes a failure
The check engine light, also called the malfunction indicator lamp, tells you the vehicle’s onboard computer has detected a problem, often one related to emissions. A smog check on most 2000-and-newer vehicles reads that computer. If the light is on, the vehicle fails, whatever the tailpipe is doing.
What to do first
- Get the code read. Many auto parts stores read codes for free, and any repair shop can do it.
- Find the cause. The code points to a system, not always a single part, so a proper diagnosis saves money.
- Repair it. Fix the underlying problem, not only the symptom.
- Drive normally. After a repair, the computer needs time to finish its self-checks. See our article on readiness monitors.
- Then take the smog test.
Common causes of a check engine light
- A loose or worn gas cap
- A failing oxygen sensor
- Engine misfires from worn spark plugs or ignition parts
- A worn catalytic converter
- Problems in the evaporative (EVAP) system
Some of these are inexpensive, and others are not. Get a diagnosis before you decide.
Do not just clear the code
Clearing the code or disconnecting the battery turns the light off temporarily, but it also resets the computer’s readiness checks. Your car can then fail for being not ready, and if the real problem is still there, the light will come back. It is not a shortcut.
If your car fails
You will receive a report that shows why. Our failed smog check help page walks through repairs, retests and where to look for help with costs.
Ready to test?
Once the light is off and your vehicle has been driven normally for a while, drive in to Blaze Smog. Walk-ins are welcome Monday to Saturday, 8 AM to 5 PM.
Sources: the California Bureau of Automotive Repair and the DMV smog check pages. Rules and fees can change, so always check your DMV renewal notice for what applies to your vehicle. This article is general information, not legal advice.
Hours
| Monday | 8 AM–5 PM |
| Tuesday | 8 AM–5 PM |
| Wednesday | 8 AM–5 PM |
| Thursday | 8 AM–5 PM |
| Friday | 8 AM–5 PM |
| Saturday | 8 AM–5 PM |
| Sunday | Closed |
No appointment needed.
More smog tips
Readiness monitors not ready: why your car can fail smog after a battery change
After a battery change or code reset, your car needs to finish self-checks before it can pass smog.
Read article
What is checked in a smog test?
Visual and functional checks, an onboard computer scan and, for older cars, a tailpipe test.
Read article
Smog check when buying or selling a car in California
Sellers generally provide the smog certificate. Newer cars and some family transfers are treated differently.
Read articleNeed a smog check? Just drive in.
No appointment needed. Open Monday to Saturday, 8 AM to 5 PM. Opening soon in the South Bay.