No. In California, a check engine light that is on during the test means the car fails. It does not matter how clean the exhaust looks or how well the car drives, because the light is the car’s own computer reporting an emissions problem.
What catches many drivers out is the next step. Turning the light off is not the same as being ready to test, and the quick fixes people try can leave a car unable to pass for days. Here is how the light, the car’s self-checks and the smog test fit together, and the order to do things in.
Why the Light Fails the Test
Most cars built since 1996 have an onboard diagnostic system that watches the engine and emissions parts and lights the check engine light, officially the malfunction indicator lamp, when something is wrong. At a smog check the technician connects to that system. The Bureau of Automotive Repair’s On-Board Diagnostic test reference lists a lamp that is commanded on while the engine is running as a failure.
A working light behaves a particular way: it comes on briefly when you turn the key and goes out once the engine starts. If it stays on with the engine running, the car fails. Our guide to what to expect at a smog check shows where this check falls in a visit.
What Usually Turns the Light On
The light can mean dozens of things, but the same few causes come up again and again before smog checks. A gas cap that does not seal lets fuel vapor escape, and the evaporative system flags it. A worn oxygen sensor throws off the air and fuel mix. Old spark plugs or a weak ignition coil cause misfires. Cracked vapor hoses or a stuck purge valve leak in the evaporative system. And a worn catalytic converter will light it with a catalyst efficiency code, which we cover in what to do about a catalytic converter and your smog check.
Only a scan of the stored code tells you which one it is, so do not guess. Replacing parts without a diagnosis is how people spend money and still fail.
If the Light Is Flashing
A steady light means get it checked soon. A flashing light usually points to an active misfire, which can damage the catalytic converter quickly. That is the standard guidance from most manufacturers, so reduce your driving and get it looked at before you think about a smog check.
Why Clearing the Code Does Not Work
Anyone with a cheap scanner can erase a trouble code and turn the light off. The problem is what else it erases. The car’s onboard system runs self-tests on its emissions parts, called readiness monitors, and clearing codes resets them to “not ready.” Disconnecting the battery has the same effect.
A car with monitors that are not ready cannot pass. BAR’s OBD test reference allows very few. For gasoline vehicles from model year 2000 on, only the evaporative system may be incomplete, and for 1996 through 1999 gasoline vehicles, any one monitor may be. Anything beyond that fails the OBD test. If the underlying problem was never fixed, the light will simply return when the system reruns its check, and you will have wasted the trip.
The monitors finish on their own as you drive, but how long it takes depends on the car and on which monitor is missing, and some need particular conditions such as a cold start or steady highway speed. BAR does not publish a set number of miles, so be wary of anyone who gives you one. If you are unsure, ask your repair shop or your technician whether the monitors are ready before you pay for a test.
The Order That Works
Start with a scan to read the stored code. Have the real cause repaired, and get the repair in writing. Then drive the car normally, in varied conditions, until the monitors are complete and the light stays off. Test last. Following this order gets most drivers to a pass in one visit, instead of a failed test followed by another.
If the test is close, do not wait. The deadline is your registration renewal, and how often you need a smog check explains when yours falls. How long a smog check takes covers the visit itself.
If the Repair Is Expensive
A failed smog check with a big repair bill is exactly what the Consumer Assistance Program is for. BAR lists repair assistance of up to $1,450 for model year 1996 and newer vehicles for income-eligible drivers whose vehicle failed its smog check, and the details are in our guide to the California Consumer Assistance Program. Apply before the repair is done.
BAR also has a repair cost waiver. Its Smog Check Reference Guide says California law limits the amount you are required to spend on repairs to pass a biennial smog check to $650, and that the waiver postpones the smog certificate requirement for up to two years. Waivers do not apply to vehicles with tampered emissions controls, and they have other conditions, so ask BAR or the Referee program before you rely on one. Our post on what happens if your smog check fails covers the steps after a failure.
Frequently Asked Questions
Will a check engine light always fail a smog check?
Yes. A lamp that is on while the engine is running fails the OBD portion of the test.
Can I clear the code myself and then pass?
Clearing the code resets the readiness monitors. Until they complete, the car cannot pass, and if the problem was not fixed, the light comes back.
Does disconnecting the battery clear the light?
It can, but it also resets the monitors, so the car will not be ready to test right away.
How far do I need to drive before a smog check?
It depends on the car and which monitor is incomplete. BAR does not set a number of miles. Drive normally in varied conditions, and ask a technician whether the monitors are ready.
My light went off by itself. Can I test?
Not necessarily. A light that goes out can still leave monitors not ready, and the problem may return. Have the stored code read first.
Can a loose gas cap really cause this?
Yes. A cap that does not seal can set an evaporative system code. Tighten it, or replace it if it is cracked, then have the code read.
Smog Check After a Check Engine Light Repair in Torrance
When the light is off and the repair is done, we can test your car. We are a STAR-certified station serving Torrance and the South Bay, so we test both regular and STAR-required vehicles. If you are not sure whether your car is ready, call us before you come in.
EZ Smog Check 1719 Abalone Ave UNIT 407, Torrance, CA 90501 Phone: 310-533-1940
Hours: Monday to Friday, 8:00 am to 5:00 pm Saturday, 9:00 am to 4:00 pm
No appointment necessary. Pass or have a free re-test.
