Check engine light is on
A 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.
How urgent is it?
Redline can’t determine from a symptom or code alone whether your car is 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 car →
Start a case with what you’re noticing. Redline weighs these causes against your car’s evidence and names the check that would settle it.
Common questions
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?
Redline can’t tell from the light alone whether driving on it is a good idea. A steady light means get the code read soon. A flashing light means an active misfire — stop driving and get it checked right away to avoid catalytic-converter damage.
Related OBD-II codes
If your car has stored a trouble code, these often come with this symptom:
Related symptoms
General guidance, not a substitute for a hands-on inspection. Cost ranges are broad reference ranges, not quotes. For safety-related issues, have the car inspected before driving.
Seeing this on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.