16 Most Common Reasons Your Check Engine Light Is On

Sometimes, the problem is as simple as remembering to screw in the gas cap after filling up. The gas cap helps keep the fuel system under the correct pressure. The fifth most common repair needed for check engine lights in 2022 was simply a new fuel cap or a tightening of the existing cap.

While some newer cars do not use spark plug wires, many older vehicles have one wire per plug. These wires should generally be replaced every 30,000 to 50,000 miles, according to the manufacturer’s specifications, and are a simple repair that can be done at home with no special tools.

A powertrain control module is a computer that manages over 100 different functions in engine and transmission control. Reprogramming the PCM is a quick task that usually takes about an hour of labor time and sometimes consists of a software update.

A car’s evaporative emissions system keeps fuel vapors from escaping into the atmosphere. The purge control valve allows these fuel vapors to be burned in the engine, but a damaged valve can cause rough idle. Replacement of this part is quick and straightforward.

An EVAP purge solenoid, also called a canister purge solenoid, is a switch on the canister that stores fuel vapors when a car is not in use until the engine is started again and the vapors can be burned. When a purge solenoid fails, it not only affects the vehicle’s emissions but can also lead to poor engine performance or difficulty starting the vehicle.

Ignition coils take electricity from the car’s battery and transform it into the higher voltage needed to ignite the spark plugs. While older cars have only one ignition coil, many modern vehicles have one coil per cylinder. If one or more of these coils fails, the engine can start to run rough, and other vehicle systems can also become damaged.

A part of the engine’s cooling system, a thermostat opens and closes as needed to regulate the coolant temperature. Sometimes, a thermostat malfunctions and is stuck in the open or closed position, causing the engine to run cooler than normal or overheat, triggering the check engine light.

A functioning oxygen sensor measures the amount of oxygen in the car’s exhaust to help a vehicle burn the right mixture of fuel and air. When the O2 sensor falters, not only does fuel economy go down, but other parts, including spark plugs and catalytic converters, can also become damaged.

Cracked vacuum hoses are the most common cause of vacuum system leaks, which can lead to a wide variety of problems, including impaired starting, poor acceleration, and a rough idle. While new vacuum hoses cost just a few dollars apiece, tracing the source of the leak can be quite time-consuming for a mechanic.

A car’s mass air flow sensor monitors the amount of air entering the engine to determine how much fuel to inject. When the MAF sensor malfunctions, the engine may stall or have problems accelerating, and fuel economy can go down by as much as 25%.

Sometimes, the check engine light can come on because of a bad tank of gas. Old fuel, contaminated fuel, or lower-grade fuel — especially if the car runs on premium — can cause poor engine performance. If bad gas is the culprit, it may be advisable to have a professional drain the tank and refill it.

When a battery starts to fail, the low voltage in the car’s computer may cause the check engine light to come on. This easy fix will cost about an hour of labor time plus the price of the new battery.

Ignition coils and spark plugs work together to ignite the fuel in the engine’s cylinders. But when spark plugs go bad, the ignition coils can become damaged, and the engine may misfire. This will lead to the need to replace both parts at once. This is a good reason to change spark plugs every 100,000 miles.

The exhaust gas recirculation system reduces nitrous oxide emissions by recirculating exhaust gas back into the engine cylinders. When an EGR valve becomes clogged or fails, the car could experience sluggish acceleration or a lack of power when going uphill. The valve may simply need to be cleaned and reinstalled, or a new valve may be needed.

A fuel injector emits a fine mist of gasoline into the engine’s combustion chamber. A common reason that these parts break is poor quality fuel, which clogs the injectors with debris or impurities. Unfortunately, new fuel injectors can run anywhere from $300 to $900, depending on the vehicle.

The most diagnosed check engine light problems are catalytic converter issues, which made up 6.1% of engine light-related repairs in 2022. Catalytic converters, required on all gas vehicles built after 1975, are a part of the exhaust system that converts toxic pollutants into less toxic emissions. Unfortunately, due to precious metals used in the converters, the price of a new part is steep.