Diagnostics guide
What Does the Check Engine Light Mean? Causes and Next Steps
Written by the ASE-certified technicians at Auto Repair Shops America · 15+ years of hands-on repair experience
The short answer
Steady vs. flashing check engine light
Since 1996 every US car uses OBD-II, which stores diagnostic trouble codes (DTCs). A steady light flags a stored fault; a flashing light signals a severe misfire that can overheat the catalytic converter.
Most common causes
Loose gas cap, failed oxygen sensors, worn spark plugs, bad ignition coils, mass airflow sensor faults, EVAP leaks and catalytic converter efficiency codes like P0420.
Why a free code scan isn't a diagnosis
A code like P0171 (system too lean) can come from a vacuum leak, dirty MAF sensor, weak fuel pump or leaking injector. Proper testing prevents throwing parts at the problem.
What to do next
Note symptoms such as rough idle or poor fuel economy, tighten the gas cap, then book a professional diagnostic.
Frequently Asked Questions
Will the light reset itself?
Sometimes after the fault clears over several drive cycles, but the code stays stored.
Can I pass inspection with the light on?
No — in states with emissions testing, an illuminated MIL is an automatic fail.
Get it fixed by a mobile mechanic
We come to you in 30+ metros including Dallas, Houston and Los Angeles.
Call 469-669-2986