Few things are as frustrating—or confusing—as a check engine light that turns on, then disappears, only to come back again later. While it might be tempting to ignore it when the light goes off, an intermittent check engine light is often your vehicle’s way of telling you something isn’t quite right. Here’s what you need to know about why this happens and what to do next.
What the Check Engine Light Means
The check engine light (CEL), also called the malfunction indicator lamp (MIL), is part of your vehicle’s onboard diagnostics system (OBD-II). It turns on when the engine control unit (ECU) detects a problem that affects emissions, performance, or engine operation.
When the light comes on and off, it usually means the system is detecting a problem that occurs only under certain conditions.
Common Reasons the Check Engine Light Comes On and Off
1. Loose or Faulty Gas Cap
This is one of the most common and least expensive causes.
- A loose, cracked, or missing gas cap can cause fuel vapors to leak.
- This triggers the emissions system and turns on the light.
- The light may turn off after several drive cycles once the cap is tightened.
Tip: Tighten the gas cap until it clicks and drive for a few days to see if the light stays off.
2. Intermittent Sensor Issues
Sensors monitor everything from oxygen levels to airflow and engine temperature.
- A sensor that is starting to fail may work sometimes and malfunction at others.
- Common culprits include oxygen sensors, mass airflow sensors, and coolant temperature sensors.
These issues often cause the light to appear under specific driving conditions, such as highway speeds or cold starts.
3. Engine Misfires That Come and Go
Minor misfires can trigger the check engine light intermittently.
- Worn spark plugs
- Failing ignition coils
- Fuel delivery issues
If the misfire becomes severe, the light may flash—this is a sign you should stop driving and seek immediate service.
4. Emissions System Problems
Components like the EGR valve, EVAP system, or catalytic converter can cause intermittent warnings.
- Temperature changes can affect emissions system performance.
- Small leaks or partial blockages may only be detected occasionally.
5. Driving Conditions or Weather Changes
Sometimes the issue isn’t constant:
- Extreme heat or cold
- High humidity
- Short trips where the engine doesn’t fully warm up
These conditions can temporarily affect sensors and emissions readings.
6. Computer Reset After Temporary Faults
If the ECU detects a problem but doesn’t see it again after several drive cycles, it may turn the light off automatically. However, the trouble code is often stored in the system’s memory and can still be retrieved with a diagnostic scan.
Is It Safe to Drive?
If the check engine light:
- Is steady: You can usually drive short distances, but schedule a diagnostic soon.
- Is flashing: Stop driving as soon as possible. A flashing light often indicates a serious issue that could damage the engine or catalytic converter.
Even if the light goes off, it’s best not to ignore it entirely.
What You Should Do Next
- Check the gas cap and tighten or replace it if needed.
- Pay attention to symptoms like rough idling, poor fuel economy, or unusual noises.
- Get a diagnostic scan—many auto parts stores offer free code scans.
- Consult a mechanic if the light continues to return or if drivability issues appear.
Final Thoughts
A check engine light that comes on and off is often an early warning sign. While it might point to something simple, it can also indicate a problem that’s gradually getting worse. Addressing the issue early can save you money, improve performance, and prevent more serious repairs down the road.
