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Is Your Check Engine Light On? Here Are 5 Common Culprits

The check engine light on a dashboard is illuminated.

As a driver, there’s no greater frustration than seeing your check engine light on for no obvious reason — or after you’ve already repaired what you thought was the issue. The real problem is that the check engine light is so ambiguous.

With the check engine light on, it’s important to note that there are some 5,000 diagnostic trouble codes (DTCs) that might be behind it. Fortunately, some DTCs are more common, and usually easier to diagnose, than others. Here are five of the most common problems that can illuminate your check engine light, along with their DTCs, which are useful if you attempt to access your on-board diagnostics system.

1. Faulty Oxygen SensorOxygen Sensor Failure Accounts for Many Check Engine Light On Problems

Oxygen sensors monitor fuel trim and catalytic converter functioning, depending on location. Uncorrected fuel trim, due to a faulty oxygen sensor or heating circuit, results in higher emissions, lower fuel economy and possible damage to the catalytic converter.

The common DTCs are P0030, P0131 and P0165.

2. Faulty Ignition

Ignition coils, spark plug wires and spark plugs are designed to deliver a precisely timed spark to burn fuel in the cylinder, but damage or weakness can lead to incomplete combustion or a misfire. A faulty ignition should be remedied immediately.

The common DTCs are P030x and P035x.

3. Failed Catalytic Converter

The catalytic converter reduces emissions from harmful compounds found in car exhaust by converting them into less harmful compounds before they leave the exhaust system. However, overheating, underheating or contamination can reduce its effectiveness and lead to your check engine light illuminating.

The common DTC for this issue is P0420.

4. EVAP Problems

To keep unburned fuel vapors from contaminating the atmosphere, a system of tubes, valves and a charcoal canister store them until they can be burned in the engine. This is called the evaporative emission control, or EVAP, system. Faulty valves are common, but refueling while the engine is running or forgetting to install the gas cap are more common, all of which can trigger the check engine light.

The common DTCs related to the EVAP system are P0440, P0446 and P0455.

5. Contaminated MAF

The mass air flow (MAF) sensor samples and measures air going into the intake system, but blockage or contamination can skew sensor readings, leading to fuel trim problems. Cleaning it might help, but sometimes the vehicle requires a replacement sensor for a sure fix.

The common DTCs related to the MAF sensor are P0171 and P0172.

Thanks to advanced electronic controls, today’s engines are more powerful, cleaner and more efficient than ever before, but everything wears out, burns out or breaks eventually. Delaying check-engine-light repairs can lead to reduced fuel economy and even collateral damage. For example, the check engine light might be on due to a spark plug or thermostat issue, but ignoring it could result in catalytic converter failure.

Even if your vehicle seems to be operating well for the moment, it’s important to resolve small issues when your check engine light comes on to avert bigger problems. There’s no need to panic, but have your vehicle scanned, diagnosed and repaired as soon as possible.

Check out all the tools & equipment available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information what might turn your check engine light on, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons.


Benjamin Jerew View All

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

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