Car symptom
Check engine light on
The light means the ECU stored a code. Steady is usually sensor/emissions; flashing is a severe misfire.
Most likely causes
- EVAP (loose fuel cap)
- Oxygen sensor aging
- Catalyst efficiency
- Misfire (plugs, coils)
- Thermostat stuck open
Checks to run
- Check the fuel cap first
- Read the stored code
- Clear and see if it returns
- Confirm before replacing
Tools you may need
OBD-II scanner
Confirm before replacing. These are hypotheses to check — never buy parts on a guess. If the fault involves brakes, airbags or fuel under pressure, use a professional.
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FAQ
It depends on the cause. Some causes are cheap and quick; others (like a head gasket or seized caliper) need attention soon. Work through the checks in order and confirm before replacing parts.
Start with the free checks listed here. If they involve high voltage, fuel under pressure, airbags or brakes, let a professional handle it.
The interactive diagnostic asks targeted questions to narrow it down, and every cause must be confirmed with a measurement — never guess.