My check engine light is on — what should I do?
The short answer
A steady check engine light means your car’s computer logged a fault — anything from a loose gas cap to a failing sensor. If the car runs normally, it’s safe to drive briefly, but get it diagnosed within days: small faults compound into expensive ones, and in Illinois emissions counties the light is an automatic test failure.
Reviewed by Joe Zeiter
Service Director · Grote service team · Updated July 17, 2026
Likely causes, ranked
Oxygen sensor or fuel-trim faults
Aging O2 sensors and lean/rich codes are the most common check-engine causes nationwide. Symptoms are subtle — mostly falling MPG — while the catalytic converter quietly absorbs the abuse.
EVAP leak — including the gas cap
The emissions system that captures fuel vapor is leak-tested by the computer constantly. A worn cap seal, cracked hose, or faulty purge valve sets the light. Sometimes it really is the gas cap.
Misfires — plugs, coils, or injectors
Worn ignition parts cause intermittent misfires that set codes before you feel them. Left alone, they escalate to the flashing-light stage that destroys converters.
Catalytic converter efficiency (P0420/P0430)
Often the end result of an ignored upstream problem. Sometimes it’s a lazy sensor mimicking converter failure — worth verifying before spending four figures.
What to do right now
- 1
Check the gas cap first — tighten until it clicks. If that was it, the light clears itself over a few days of driving.
- 2
Pay attention to how the car runs. Normal = book a diagnosis this week. Rough, down on power, or flashing = stop and call now.
- 3
Skip the parts-store code read → parts-cannon cycle. Codes are symptoms; diagnosis finds causes. Ours is $89–$149 and applies to the repair.
What it typically costs
Diagnosis at Grote Auto Service is $89–$149, credited toward the repair. Fix costs vary wildly by cause — a $40 gas cap, a $250 sensor, or a converter at $900–$2,400 that proper early diagnosis usually prevents.
Our diagnosis fee goes toward the repair.
Related questions
Can I just clear the code and see if it comes back?
You can — but clearing also erases the freeze-frame data that helps diagnose it, and in Illinois a recently-cleared computer fails emissions as “not ready.” Better: get it diagnosed, then clear it once it’s actually fixed.
The light has been on for a year and the car runs fine. Do I really need to fix it?
Two problems: an active fault often has the computer running compensations that waste fuel and wear the converter, and the lit light will mask the next fault — the serious one gets no announcement. Plus it’s an automatic emissions failure.
What actually fixes it
If it turns out to need real work, this is what usually does it — with pricing published up front, so there are no surprises.
Prefer we reach out?
We respond within the hour during business hours — usually faster.
This guide describes common patterns, not a diagnosis of your specific vehicle — that takes an inspection. If your symptom is marked urgent, treat it as urgent. When in doubt, stop driving and call (779) 888-5000.
Stop wondering. Start knowing.
A proper diagnosis costs less than a wrong guess — and ours is applied to the repair.
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