No. In California, a car with the check engine light on fails the smog check, even if it seems to run fine. The light means the engine computer has found a fault that can affect emissions, and the inspection counts that as a failure. The good news is that the fix is often simple once the real cause is found, so the smart move is to diagnose first and test second.
Many different faults turn on the same light. Some of the ones we see most before a smog check are:
A code points to the system that has a problem. It does not always name the part. That is why testing matters before anything gets replaced.
Cars from 1996 and newer have an onboard computer, called OBD-II, that keeps watch over the engine and emissions parts. When it sees a problem, it stores a trouble code and turns on the check engine light. During a California smog check, the station plugs into that computer. If the light is commanded on, the car fails, no matter how clean the tailpipe looks that day.
When you turn the key to "on" before starting, the check engine light should glow for a moment and then go out once the engine runs. That short glow proves the bulb works. If it never comes on at all, the inspector cannot trust it, and that is also a fail. A light that stays dark can hide a problem, so it is worth checking.
The computer runs its own self-tests, called readiness monitors, while you drive. After a dead battery, a battery change, or someone clearing the codes, those results are wiped and the tests must run again. Until enough of them finish, the car reports "not ready," and the smog check cannot pass. This is why turning the light off the morning of the test does not work. The fix is normal driving over a few days, or finding out why a self-test will not finish.
If your light is on and your registration is due, do not just book a test and hope. Have the code read and the cause diagnosed. Once the repair is done, the car may need some normal driving so the self-tests can finish. Then it is ready for the smog check. If the light is flashing, the engine has a serious misfire. Drive as little as possible and have it checked right away, because a flashing light can mean damage to the catalytic converter.
123 Auto Center is a BAR-licensed smog check and repair center, so we can find the problem, fix it, and run the test in one place. Our smog check is $46.75 + certificate. Our diagnostic rate is $145/hour, and the first hour is applied toward your repair when the total is over $250. If your car fails, California programs may help with the cost of emissions repairs. See our smog check page for details, and our engine diagnostics page for how we track down warning lights. If your car also shakes at a stop, read rough idle or stalling: common causes, or see our full engine performance and drivability guide.
Schedule a Smog Check Call (909) 527-2525
Can I pass smog if the check engine light comes on and then goes off?
Only if the light is off and the computer has no active fault when you test. If the problem is still there, the light often comes back, and the computer may report its self-tests as not ready. It is better to find the cause first.
Will clearing the codes help me pass?
No. Clearing codes, or disconnecting the battery, also resets the computer's self-tests. They show as not ready until the car is driven enough to finish them, and too many not ready results is a fail.
Can a loose gas cap turn on the check engine light?
Yes. A cap that does not seal can set a fuel vapor (EVAP) system code. Tighten it until it clicks. If the light stays on after a few days of normal driving, have it checked, because other EVAP leaks set the same kind of code.
How much does a smog check cost at 123 Auto Center?
Our smog check is $46.75 plus the certificate. If the check engine light is on, we can diagnose the cause first, so you do not pay for a test the car would fail.
