The Check Engine Light (CEL) is never good news about your car. At a minimum, something as simple as your gas cap may be loose, to a failed sensor, wiring, connector, or worse yet, a larger engine problem. Ignoring it if the car runs and drives fine may seem reasonable for some time, but problems will continue to mount. Here at AutoZone, we’re pretty familiar with the light, so we’ve rounded up the 5 most common causes and everything else you need to know when you’re wondering, “Why is my Check Engine Light on?”
IS THE CEL LIT OR FLASHING?
If the light is steadily lit, that means that one of your engine sensors is doing one of two things – it’s either sending a signal to the vehicle’s ECU that something is wrong, or the vehicle’s ECU (computer) itself has lost an accurate read on that particular sensor – meaning, the sensor has gone out. Sometimes, multiple sensors send signals, indicating clues that something bigger has gone awry. You can still drive with the light illuminated, assuming the car is running and driving fine, but you should get it checked out as soon as possible. If the light flashes, that means you need to pull over immediately as your vehicle has a serious issue that could cause imminent damage.
CAN I CHECK MY OWN LIGHT?
You can. All that you need to check your own light is an OBD-II reader, which can pull codes that help you find out what’s wrong with your car. If you don’t have a code reader, keep reading to learn the most common causes, and then bring your car to your local AutoZone to find out why your Check Engine Light is on.
5 REASONS YOUR CHECK ENGINE LIGHT MAY BE ON.
FAILED OXYGEN (O2) SENSORS.
1_ Your oxygen sensor measures the amount of unburned oxygen in your vehicle’s exhaust system.
Oxygen sensors also live a hard life in +800 degree exhaust temps and are prone to fail once over 80,000 miles. It’s important to note here that there are a multitude of engine codes where the oxygen sensor is telling us something is wrong, but that DOES NOT mean that the oxygen sensor is bad. Be careful determining that a sensor is truly at fault, as opposed to determining something else is happening. This cannot be more apparent than in the ever-popular PO420 / PO421 engine code, which is detailed HERE.
What happens if I don’t replace it?
Oxygen sensors also live a hard life in +800 degree exhaust temps and are prone to fail once over 80,000 miles. It’s important to note here that there are a multitude of engine codes where the oxygen sensor is telling us something is wrong, but that DOES NOT mean that the oxygen sensor is bad. Be careful determining that a sensor is truly at fault, as opposed to determining something else is happening. This cannot be more apparent than in the ever-popular PO420 / PO421 engine code, which is detailed HERE.
What happens if I don’t replace it?
- Your engine will burn more fuel than needed
- You will lose fuel economy, and get fewer miles per gallon
- Faulty sensor(s) can cause damage to your spark plugs and catalytic converter
GAS CAP IS LOOSE, DAMAGED, OR MISSING – EVAP.
2_ Your gas cap is part of an entire, sealed system that recirculates gasoline vapors from your gas tank and keeps them from escaping into the air. Because of this, this system of lines and valves is closely monitored, and one of the key sealing pieces of this is the gas cap. If you leave your gas cap off accidentally, you’ll notice this light rather quickly. These leaks in the gas tank recirculation (also known as EVAP emission control) system are often grouped as “small” or “large” leaks. Its important to note that the problem is not always the gas cap, but could be one of the plastic recirculation lines, fittings, or connectors causing the leak, but many times, the gas cap is the culprit.
What happens if I don’t replace it?
What happens if I don’t replace it?
- You can lose fuel through evaporation
- Your recirculation system will not function properly
- You will have to stare at the CEL until it’s fixed
BAD CATALYTIC CONVERTER.
3_ Your catalytic converter helps protect our environment by converting harmful carbon monoxide into carbon dioxide. Your downstream oxygen sensors carefully monitor the converter’s performance, and if something goes awry, a number of trouble codes based on “Catalyst efficiency” will appear. It’s important to note that a catalytic converter will more than likely not fail on its own. Generally, something causes it to fail, or not perform properly. Because of this, simply replacing the converter or the oxygen sensors around it will often not fix the problem, and often leads to yet another failed converter. Things as serious as a blown head gasket can force burned coolant vapor into the exhaust, triggering a catalyst engine code.
What happens if I don’t replace it?
What happens if I don’t replace it?
- Your vehicle will not pass an emissions test
- You will experience reduced performance and fuel economy
- Whatever caused the converter to fail initially will continue to get worse, possibly causing engine failure
MASS AIRFLOW SENSOR OR INTAKE TRACT
4- Your mass airflow sensor (MAF) measures the amount of air entering the engine to determine how much fuel is needed to run your engine properly. If there are any leaks before or after the mass airflow sensor in the intake tract, a light can trigger. Mass airflow sensors are sensitive to oil, dirt, and water vapor, and any contamination on them can trigger a light. Sometimes, cleaning the sensor with intake / mass airflow sensor cleaner can fix the issue. Other times, the sensor needs to simply be replaced. Be sure to check the intake ducting carefully for leaks, rips or tears/damage.What happens if I don’t replace it?
- A faulty MAF sensor can cause rich or lean conditions, which can lead to further failures of other parts
- You will experience reduced performance and fuel economy
MISFIRES.
5- Your spark plugs ignite the air/fuel mixture in the combustion chamber of your vehicle, but if the timing of that spark, the mixture of the fuel and compression of the air/fuel isn’t perfect, you will get a misfire. Multiple issues can cause a misfire either on one particular cylinder, or a “multiple cylinder misfire” meaning, the misfire is random.
- Defective ignition coil: (either DIS or coil-on-plug). A quick trick to determine if a coil is causing a misfire is to simply move the coil and swap with another on the engine. If you have a #2 misfire and move the #2 coil to #4, for example, then the misfire moves to #4, you know the coil is suspect.
- Defective fuel injector: If a fuel injector fails to deliver the proper amount of fuel to a cylinder, you will have a misfire. Unlike swapping the ignition coil with a neighboring coil, a fuel injector is more difficult to move.
- Vacuum leak: Vacuum leaks often cause multiple cylinder misfires, or lean running conditions. They also can be very difficult to diagnose.
- Worn spark plugs and/or wires: Sometimes, spark plugs can go far past their life cycle, and fail, causing a misfire. On vehicles with spark plug wires, a defective wire or wires can also cause this.
- Bad compression: More serious, a burnt or bent valve, worn piston rings, or valve timing that is off will cause a misfire as well. These issues will all become apparent during an engine compression test.
- You will experience poor performance (reduced power, engine missing) and reduced fuel economy
- The misfire will only get worse, and will lead to mechanical failure in the engine
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