Urgency: Moderate
Check Engine Light
Investigate soon. Driving short distances is generally okay, but book a diagnostic check.
What the Check Engine Light Means
The check engine light on a your car is driven by the OBD-II system: when the powertrain control module sees a sensor reading it cannot reconcile, it stores a diagnostic trouble code and lights the symbol. It covers everything from emissions to fuel and ignition, which is why it is the most misunderstood light on the dashboard.
Common Causes
- Loose or faulty gas cap
- Failing oxygen (O2) sensor
- Worn spark plugs or ignition coils
- Dirty mass airflow (MAF) sensor
- Catalytic converter efficiency loss
- Vacuum or intake leak
How to Fix It
- Check the fuel filler cap is clean and clicks tight
- Scan for DTCs with an OBD-II reader
- Note whether the light is steady or flashing
- Address the specific code (e.g. replace a failing coil or O2 sensor)
- Clear the code and complete a drive cycle to confirm the fix
Related DTC Codes
| Code | Meaning |
|---|---|
P0011 | Camshaft Position Timing Over-Advanced (Bank 1) Variable valve timing on bank 1 is over-advanced, often from low oil pressure or a stuck VVT solenoid. |
P0101 | Mass Airflow Sensor Range/Performance The MAF sensor reading is out of expected range, commonly from contamination or an intake leak. |
P0128 | Coolant Thermostat Below Regulating Temperature The engine is not reaching normal operating temperature, usually a stuck-open thermostat. |
P0171 | System Too Lean (Bank 1) The air-fuel mixture on bank 1 is too lean, frequently due to a vacuum leak or a dirty mass airflow sensor. |
P0300 | Random/Multiple Cylinder Misfire Detected The engine control module detects misfires across more than one cylinder, often from ignition, fuel, or vacuum faults. |
P0301 | Cylinder 1 Misfire Detected A specific misfire in cylinder 1, commonly caused by a failing coil, spark plug, or injector. |
P0420 | Catalyst System Efficiency Below Threshold (Bank 1) The catalytic converter on bank 1 is no longer cleaning exhaust efficiently, or the downstream O2 sensor is faulty. |
P0442 | EVAP System Leak Detected (Small Leak) A small evaporative emissions leak, very often a loose or worn fuel filler cap. |