Deciphering the code
Some drivers might confuse the service required or maintenance required light on the gauge cluster for the check engine light. These warning lights are unrelated. The service required light just means the car is due for an oil change or other routine care. It is not an indicator of trouble like the check engine light is.
Your local mechanic can usually diagnose the problem for about $75. But there's a way to preview what the problem might be. Do-it-yourselfers can buy inexpensive code readers from an auto parts store or online that connect to the onboard diagnostics (OBD) port and search for the code's meaning on websites such as Engine Light Help. Modern systems will display the code in an app on your smartphone.
How to turn off the check engine light
Most code readers will allow you to turn off or reset the check engine light. But this action alone does not actually address the underlying problem. In many cases, the light will simply come back on later.
With the code and its meaning in hand, a do-it-yourself interpretation can still be a little tricky — even if you are mechanically inclined, as we found out from one of our colleagues.
His wife's car started running poorly, and there was a check engine light. His code reader detected a code for the cam angle sensor. He thought about buying the sensor and installing it himself. But if he had, he would have wasted time and money because it turned out that the sensor was fine. Instead, mice had gotten under the hood and had chewed some of the wires leading to it.
What could cause the check engine light to come on?
CarMD, an automotive telematics company, published a list of the 10 most common check engine codes in 2021, along with their estimated cost of repair:
1. Replace catalytic converter(s) with new catalytic converter(s): $1,383.29
2. Replace faulty oxygen sensor(s): $243.61
3. Replace ignition coil(s) and spark plug(s): $389.43
4. Replace mass airflow sensor: $336.37
5. Inspect for loose fuel cap and tighten or replace: $25.01
6. Replace ignition coil(s): $214.83
7. Replace evaporative emissions (EVAP) purge control valve: $142.10
8. Replace fuel injector(s): $448.41
9. Replace thermostat: $236.33
10. Replace evaporative emissions (EVAP) purge solenoid: $148.81
Occasionally, the check engine light comes on when nothing is wrong with the car, said Steve Mazor, retired chief automotive engineer for the Auto Club of Southern California. It could be a temporary problem caused by a change in humidity or other factors. In such cases, the light should go off by itself after a short time.
Don't ignore that light
Mazor says that some people freak out when they see the check engine light. "They just put a piece of black tape over the dashboard light and keep driving," he said. But Mazor adds it's important to address problems indicated by the light promptly. Ignoring them could lead to larger, costlier problems later.
If the light comes on, Mazor suggests the driver check the gas cap. A loose gas cap sends an error message to the car's computer, reporting a leak in the vapor recovery system, which is one aspect of a car's emissions system. If the fuel cap is loose, tighten it and continue driving. Even so, it will take some time for the light to go off, he says.
What should you do if the check engine light comes on and it's steady rather than flashing? The most obvious answer is to get the engine checked by a mechanic. But many people do nothing, perhaps fearing an expensive repair bill. Some drivers with older vehicles may want to squeeze out as many remaining miles as possible without visiting a service garage. But before they can pass their state's vehicle inspection, they have to get the light turned off. And a state inspection is a good motivator for dealing with the problem. Ultimately, the engine and the emission control system are so interlinked that the health of the emission control system is a good indication of the general health of the car's engine.
Good backup plan
Mazor says that an inexpensive check engine code reader could be useful for car owners even if they aren't mechanically inclined.
"If the mechanic gives you the same information, at least you know they are going down the right road," he notes. A code reader can provide car owners with one more data point to help them talk with their mechanic and avoid costly or unnecessary auto repairs.