Why Is the Check Engine Light Flashing?

A flashing CEL indicates a serious drivetrain problem that could result in irreversible engine, catalytic converter or transmission damage. Stop driving the car. However, it is possible that the computer may allow you to make it to the repair shop or home by engaging “Limp Home Mode” that reduces engine power to protect the drivetrain from further damage caused by missing or out-of-parameter data.

Potential Causes of a Check Engine Light

Here are some of the most common reasons your check engine light is on:

Loose, damaged or missing gas cap. An integral component of the fuel evaporation system (as well as keeping dirt and debris from getting in the gas tank), gas cap seals wear out. Try retightening, but consider replacing it every seven years.
Failed or failing O2 sensors. A critical input sensor that effects air/fuel delivery, these sensors fail due to contaminants (bad fuel, raw fuel, oil silicates) that enter the exhaust.
Spark plugs / Spark plug wires / Ignition coil issues. All can cause engine misfires, which often result in a flashing CEL. Worn, wrong or cracked spark plugs; worn, damaged or shorted spark plug wires; an open, shorted or damaged ignition coil(s) result in incomplete combustion, allowing raw, unburned fuel to exit the combustion chamber. Once in the exhaust system, raw fuel can seriously damage the O2 sensor and catalytic converter.
Catalytic converter failure. Usually the result of raw fuel, oil or antifreeze entering the exhaust system. A bad catalytic converter can cause the CEL to come on. Plus it can result in drivability issues, including lack of power and acceleration.
Mass Air Flow (MAF) Sensor. A contaminated or failed MAF cannot accurately measure the amount of air flowing into the combustion engine and will trigger the CEL. This results in various drivability problems, or possibly a no-start condition.
Aftermarket Accessories (Alarms, Light Kits, Sound Systems). Poorly-installed aftermarket accessories can cause mayhem with sensitive computer systems. They can trigger the CEL by draining the battery, or if wires are spliced into critical emission systems.

How to Reset/Turn Off the Check Engine Light

Sometimes the CEL will simply go off by itself. That does not mean there isn’t a problem. To avoid possible expensive repairs, take your vehicle in for service to have the trouble codes read, or purchase a code reader/scan tool and plug it into the diagnostic port. You can search for trouble codes online to help identify the issue. Once fixed, a scan tool may have the ability to “clear” the code(s), turning off the CEL. Although there are procedures that may clear trouble codes and reset the CEL without a scan tool, this is something best left to the pros.