The check engine light (officially the malfunction indicator lamp) comes on when the engine computer detects a fault that could affect emissions or engine health and stores a trouble code. A solid light means a fault is stored and you should have it read soon. A flashing light means an active, severe misfire that can overheat the catalytic converter, so ease off and stop driving as soon as it is safe.
Solid vs flashing check engine light
| Light | What it means | What to do |
|---|---|---|
| Solid | A fault is stored; the engine may run normally | Read the code within a few days; watch for symptoms |
| Flashing | Severe misfire happening now | Reduce speed and load, avoid hard acceleration, stop and diagnose soon |
| On with red warnings (oil, temperature) | A serious mechanical problem may be under way | Stop the engine safely and check oil level and coolant |
Common causes
- Loose or worn fuel cap: one of the most common causes of a solid light; tighten it until it clicks and the light may clear after several drives.
- Misfires from spark plugs or ignition coils; see engine misfire and the P0300 code.
- Lean or rich mixture from a vacuum leak, a dirty MAF sensor or fuel delivery problems, which set codes such as P0171.
- Oxygen sensor faults, such as P0135, one of the most common sensor codes.
- Thermostat stuck open (P0128) and other cooling faults.
- Timing and oil problems on variable valve timing engines (P0011, P0016).
- Transmission faults, which often show up only as P0700 on a basic code reader.
How to read the code
Every car and light truck sold in the US since the 1996 model year has an OBD-II port under the dash, usually near the steering column. Plug in a scan tool, turn the key on, and read the stored codes and freeze-frame data. Many auto parts stores will read codes at no charge. Our OBD2 codes list explains how codes are built and links a guide for each common code.
How to reset the check engine light
- Fix the cause first. Clearing a code does not repair anything, and the light comes back if the fault is still there.
- Clear it with a scan tool using the clear codes or erase function.
- Or let it clear itself: after many faults are fixed, the computer turns the light off after several consecutive drive cycles without seeing the fault again.
- Disconnecting the battery also clears codes on most vehicles, but it erases learned idle and fuel settings and radio presets, and the engine may idle unevenly for a while.
Clearing codes also resets the readiness monitors, which need some normal driving to complete again, so do not clear codes just before a state inspection.
Can you drive with the check engine light on?
With a solid light and no other symptoms, short-term driving is usually fine while you get the code read. With a flashing light, overheating, low oil pressure, knocking or a major loss of power, stop: continuing can turn a sensor or ignition repair into engine damage. If a code points to serious internal trouble, see how to tell if an engine is blown and compare engines by code on our engines page.
More guides: how to use an OBD2 scanner · service engine soon light · drive cycle and readiness monitors · check engine light flashing.
Trouble codes: P0650 Check Engine Light Circuit · U0001 High Speed CAN Bus · P0603 Control Module KAM Error · P0602 Control Module Programming Error · P2610 ECM Engine Off Timer · all OBD2 codes.
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