The check engine light—it's one of the most misunderstood and anxiety-inducing symbols on your dashboard. But it's simply your car's way of telling you that its onboard diagnostic system has detected a problem. Here are the top five most common culprits and what you should do when that light appears.
First: Is the Light Steady or Flashing?
This is the most important thing to know.
- Steady Light: This indicates a non-emergency issue. You should get it checked out soon, but you don't need to pull over immediately.
- Flashing Light: This signals a serious problem, usually a severe engine misfire that is dumping unburned fuel into the exhaust system. This can rapidly destroy your catalytic converter (a very expensive repair). If your light is flashing, pull over as soon as it is safe and have your vehicle towed.
Top 5 Reasons for a Check Engine Light
1. Loose or Faulty Gas Cap
This is the best-case scenario. The gas cap is a crucial part of your car's evaporative emissions (EVAP) system. If it's loose or the seal is cracked, fuel vapors can escape, triggering the light.
- The Fix: Try tightening your gas cap. If the light doesn't turn off after a few days of driving, you may need a new cap, which is an inexpensive part.
2. Failing Oxygen (O2) Sensor
An oxygen sensor measures the amount of unburned oxygen in your car's exhaust. This data helps the engine's computer adjust the fuel-to-air mixture for maximum efficiency. A failing sensor means the computer is getting bad data, which leads to poor fuel economy and increased emissions.
- The Fix: This is a common repair. A mechanic can test the sensor and replace it if necessary.
3. Worn Spark Plugs or Ignition Coils
Spark plugs ignite the fuel in your engine's cylinders, while ignition coils provide the electricity for them to do so. If either of these components is failing, it can cause an engine misfire, which will trigger the check engine light.
- The Fix: Spark plugs are a routine maintenance item and should be replaced according to your owner's manual (typically every 30,000 to 100,000 miles).
4. Failing Catalytic Converter
The catalytic converter is a vital part of your emissions system, converting harmful gases into less harmful ones. It usually only fails if another problem—like a bad O2 sensor or worn spark plugs—is ignored for too long.
- The Fix: This is one of the most expensive repairs. The best solution is prevention: address the root cause of a check engine light promptly to protect your catalytic converter.
5. Malfunctioning Mass Airflow (MAF) Sensor
The MAF sensor measures the amount of air entering the engine, helping the computer calculate how much fuel to inject. If it's dirty or failing, it can cause rough idling, poor acceleration, and bad fuel economy.
- The Fix: Sometimes the sensor can be cleaned, but often it needs to be replaced.
Frequently Asked Questions
Q: How can I find out what's wrong myself?
A: You can buy an inexpensive OBD-II scanner online or from an auto parts store. You plug it into a port under your dashboard, and it will give you a "trouble code" (e.g., P0420). You can then look up that code online to understand the specific issue your car is reporting.
Q: Can I just disconnect the battery to turn the light off?
A: While this might temporarily turn the light off, it does not fix the underlying problem. The light will inevitably come back on, and you will have erased valuable diagnostic data that your mechanic could have used.
Q: Will my car fail inspection with the check engine light on?
A: In most states, yes. An illuminated check engine light is an automatic failure for an emissions or state safety inspection.