Symptoms / Check engine light is on
Check engine light is on
Get it checked soonA steady check-engine light means the car stored a trouble code — anything from a loose gas cap to a misfire.
What this usually means
The check-engine light (CEL) turns on when your car’s computer detects a reading outside its expected range and stores a diagnostic trouble code. A steady light means “something needs attention soon”; a flashing light means an active misfire that can damage the catalytic converter, and you should ease off the throttle and get it checked right away. The light itself doesn’t tell you the cause — you (or a shop) have to read the stored code first, then work out which of several possible faults is actually behind it.
Most likely causes
- highLoose or failed gas capA cap that isn’t sealing lets the evaporative-emissions system detect a leak. It’s the cheapest and most common trigger — always worth checking first.
- highFailing oxygen (O₂) sensorA worn O₂ sensor reports incorrect exhaust readings, hurting fuel economy and emissions. Common on higher-mileage cars.
- mediumEngine misfireWorn spark plugs, coils, or injectors can cause a cylinder to misfire — often felt as a rough idle alongside the light.
- mediumCatalytic converter efficiency below thresholdOften the downstream result of a long-ignored O₂ sensor or misfire rather than a converter that simply “wore out.”
- lowMass airflow (MAF) sensor faultA dirty or failing MAF sensor mis-measures incoming air, causing poor running and a stored code.
Is it safe to drive?
Typical fix & cost
The fix depends entirely on the stored code. A gas cap is a few dollars; an O₂ sensor or spark-plug-and-coil job is a moderate repair; a catalytic converter is the expensive end. The key is to read the code and confirm the real cause before replacing parts — a lot of money is wasted swapping converters when the actual fault was a cheap sensor.
Typical range: $20–$1,200
A gas cap is near-free; sensors and plugs are moderate; a catalytic converter is the high end.
The price depends on which cause it turns out to be — so confirm the cause before paying. Diagnose this for my exact vehicle →
Frequently asked
Can I just reset the light?
Clearing the code without fixing the cause only turns the light off temporarily — it will return once the computer re-detects the fault, and you’ll have lost the stored data that helps diagnose it.
Is it safe to drive with the light on?
A steady light usually yes, short-term. A flashing light means an active misfire — drive gently and get it checked promptly to avoid catalytic-converter damage.
Seeing this on your car? Get a diagnosis specific to your exact year, make and model — RedlineAi ranks the likely causes against real recall and complaint data, with an honest confidence score.
Diagnose my vehicle →Related OBD-II codes
If your car has stored a trouble code, these often accompany this symptom:
Related symptoms
This is general guidance, not a substitute for a hands-on inspection. Cost ranges are broad estimates to set expectations, not quotes. For safety-related issues, have the car inspected by a licensed mechanic before driving.