Five reasons your check engine light turned on and how much it could cost you

Americans are holding onto their vehicles longer than ever, with the average age of a car in 2020 rising to a record 12.1 years, according to data from IHS Markit. But if you own a car for long enough and something goes wrong mechanically, you're likely to see an amber check engine light somewhere in the gauge cluster. Formally known as the malfunction indicator lamp, it is a signal from the vehicle's engine computer that something's wrong.

The light could be a minor issue, such as a faulty gas cap, or it could mean something more serious, such as a misfiring engine. In many cases, it means that you'll need to visit a repair shop to diagnose and repair the issue.

CarMD, an automotive telematics company, published a list of the most common check engine codes in 2020, along with their estimated cost of repair. With this in mind, we decided to take a look at the most common check engine light-related issues ranked by frequency to give you a sense of what can go wrong in your car.

1. Your catalytic converter may need to be replaced

A catalytic converter is an emissions control system that reduces pollutants from a vehicle's exhaust. CarMD says that a catalytic converter usually won't fail unless a related root cause -- such as a faulty spark plug -- is ignored for too long. But eventually, the catalytic converter will give way with age, as 2006 model year vehicles were the most likely to need a new one. Symptoms of a faulty catalytic converter are sluggish acceleration, dark exhaust smoke and diminished fuel economy. Unfortunately, global prices for the precious metals used in the converters have soared in 2021, and replacement catalytic converters can be quite expensive.

Average cost to repair in 2020: $1,383.29

2. Your Oxygen Sensor (O2) may be faulty 

The oxygen or "O2" sensor determines how much oxygen is in the exhaust, so the engine can adjust the amount of fuel being used to run at maximum efficiency. This part will wear out with time, and neglecting oil changes can be a factor. Symptoms of a bad sensor are a rough idle, reduced acceleration and poor fuel economy.

Average cost to repair in 2020: $243.61

3. Check your ignition coil and spark plugs 

The ignition coil in an engine transforms the battery's 12-volt charge into thousands of volts, which are needed to create an electric spark to ignite the fuel. Meanwhile, spark plugs provide the spark needed to ignite the air and fuel mixture. Spark plugs will wear out over time, roughly between 60,000 and 100,000 miles depending on the vehicle. Worn spark plugs can cause engine misfires, reduced fuel economy, and difficulty starting the engine. If you have a bad spark plug and ignore it long enough, it will exacerbate the wear and tear on the ignition coil, and eventually they will need to be replaced in tandem.

Average cost to repair in 2020: $389.43

4. The mass airflow sensor may need to be replaced 

The mass airflow sensor measures the amount of air coming into the engine and determines how much fuel is needed to inject into the combustion chamber. Symptoms of a bad mass airflow sensor are jerky acceleration, engine stalling and reduced fuel economy. Last year, this repair was most common on 2012 model year vehicles.

Average cost to repair in 2020: $336.37

5. Tighten or replace your fuel cap 

This is the least expensive repair and can happen to anyone who spaced out at the gas station. If you didn't tighten the gas cap properly or forgot to put it back on, vapors can escape, which can cause the vehicle's evaporative emissions system to trigger the check engine light. A missing or loose fuel cap may also reduce fuel economy and allow harmful emissions.

Average cost to repair in 2020: $25

How to read your check engine light

The trouble with a check engine light is that it requires an extra step to determine the problem. Any mechanic should be able to diagnose the problem and rates will vary, but there's also a way to preview what the issue might be. There are inexpensive code readers that connect to the onboard diagnostics (OBD) port and will give you the fault code. Less expensive systems will only read the code, at which point you'd need to search online for what it means. The more modern code readers will explain what the issue is via a smartphone app or a more detailed interface. This information gives you a reference point when speaking with your mechanic, to avoid costly or unnecessary repairs.


Many of the common check engine light symptoms involve reduced fuel economy. This should motivate folks to take their vehicle to a mechanic sooner rather than later since it could cost them in more ways than one.