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What Causes a Cars Check Engine Light to Go On?
Few things cause more stress for drivers than seeing the check engine light suddenly appear on the dashboard. This warning light can signal anything from a loose gas cap to a major engine problem. While the causes vary, there are a few common issues that trigger it more often than others.
In this article, we’ll break down the five most common reasons for a check engine light: oxygen sensor failure, catalytic converter problems, mass airflow sensor issues, bad spark plugs, and even something as simple as a loose gas cap.
1. Faulty Oxygen Sensor
Your car’s oxygen sensor (O2 sensor) monitors how much unburned oxygen is in the exhaust and helps the engine computer adjust the fuel-air mixture.
When the sensor is dirty or broken, the ECU gets bad data. This can reduce fuel efficiency, increase emissions, and cause rough running. Most cars have between two and four oxygen sensors, and a code from an OBD-II scanner will tell you which one has failed.
If ignored, a faulty O2 sensor can damage your catalytic converter, leading to repairs that cost hundreds or even thousands of dollars.
Read more about oxygen sensors and your cars check engine light.
2. Catalytic Converter Failure or Working Below Efficiency
The catalytic works by converting carbon monoxide and nitrogen oxides into less harmful compounds. When it’s working properly, your car runs cleaner, passes emissions tests, and maintains good fuel economy.
But if the converter fails, you may notice poor acceleration, sluggish performance, and reduced fuel efficiency. The check engine light is usually triggered by an efficiency code like P0420, detected by the oxygen sensors before and after the converter.
Catalytic converters don’t fail on their own — they’re often damaged by other issues like misfires, bad spark plugs, or faulty oxygen sensors. Unfortunately, replacement costs can run from hundreds to thousands of dollars, making this one of the more expensive check engine light repairs.
Faulty or failing oxygen sensors can also cause the catalytic converter to work below efficiency. With engine codes P0420 and P0430 this is typically caused by issues not related to catalytic converter failure.
Read more about catalytic converter failure and its causes.
3. Mass Airflow Sensor Problems
The mass airflow sensor (MAF) measures how much air enters the engine and tells the ECU how much fuel to inject. A dirty or failing MAF sensor can throw off this balance, causing:
- Poor fuel economy
- Stalling
- Rough idling
- Increased emissions
MAF sensors commonly fail due to dirty or neglected air filters. Replacing the air filter once a year can help prevent sensor problems. You can technically drive with a bad MAF sensor for weeks or even months, but the car will use more fuel and eventually stall. Replacement usually costs a few hundred dollars, with most of that cost going toward the part rather than labor. Sometimes the MAF sensor is just dirty. You can clean the sensor with MAF cleaner, which can sometimes solve issue of poor sensor performance.
Where to Find the Mass Air Flow (MAF) Sensor in Your Car
On most modern fuel-injected vehicles, the Mass Air Flow (MAF) sensor is located in the engine’s air intake system. You’ll usually find it positioned between the air filter housing and the intake manifold. The sensor is typically inserted into the top of the large air intake tube that connects the air filter box to the throttle body.
To identify it, look for a small sensor with a wiring harness plugged into it, often secured with clamps or screws.
4. Worn or fouled Spark Plugs
Spark plugs ignite the air-fuel mixture in your engine, while ignition coil (or spark plug wires in older cars) deliver the electricity needed to create the spark.
When spark plugs wear out, they misfire, which feels like a jolt or hesitation when you accelerate. Misfires increase emissions, hurt fuel efficiency, and can damage the catalytic converter over time.
- Older vehicles (pre-1996) usually need spark plugs replaced every 21,000–26,000 miles.
- Newer cars may last up to 85,000 miles before needing replacement.
A fouled spark plug occurs when the tip becomes coated with foreign material like carbon, oil, or coolant. This buildup prevents the spark plug from properly igniting the fuel-air mixture, often leading to engine misfires, rough idling, poor acceleration, and reduced fuel efficiency.
Spark plug fouling can be caused by several underlying issues, including a rich fuel mixture, clogged or dirty air filter, worn piston rings, leaking valve seals, or even a coolant leak.
If your check engine light is flashing due to misfires, don’t ignore it. Replacing spark plugs is a relatively low-cost fix compared to the expensive repairs caused by neglect.
5. Loose or Broken Gas Cap
Believe it or not, one of the simplest reasons for a check engine light is a loose or broken gas cap. The gas cap helps seal the fuel system and maintain pressure.
If it’s loose, cracked, or missing, vapors escape and trigger the EVAP system, causing the check engine light to come on.
The fix is easy: make sure the cap is tightened properly after pumping gas or replace it if it’s damaged. Skipping this step could lead to wasted fuel and increased emissions. Still, it’s one of the cheapest and quickest fixes for the check engine light.
Final Thoughts
The check engine light is your car’s way of telling you something is wrong. Whether it’s a simple gas cap or a failing catalytic converter, ignoring the warning can lead to bigger and more expensive problems.
Staying on top of maintenance — such as replacing air filters, spark plugs, and oxygen sensors on schedule — can prevent most of these issues from happening in the first place.
At Los Angeles Cash for Cars, we buy cars in running driving condition — even if the check engine light is on. We service, Venice, Santa Monica, Marina del Rey, Culver City, and Beverly Hills, paying top cash for cars, trucks, SUVs, and classics.
Call (310) 893-0162 for a free quote and same-day cash offer.