Check Engine Light Diagnosis & Testing
What is a check engine light?
The check engine light will illuminate on your dashboard when your onboard computer system has identified an issue that could cause an increase in emissions from the exhaust system.
How does a check engine light work?
The computer that operates the engine takes in data from numerous sensors and controls the fuel delivery and ignition system based on that data. The computer monitors the signals coming in and going out. When it measures an anomaly it will register this as a fault, store a diagnostic trouble code in the computer, and turn on the Check Engine Light.
Related symptoms
When the Check Engine Light comes on, you may experience engine performance issues such as poor acceleration, rough idling, and an engine that won't start. In some cases, no abnormal symptoms will be experienced. Whether you experience performance problems or not, a flashing CEL indicates that an engine misfire has occurred. If you continue to drive the vehicle, you may damage the catalytic converter.
What are the symptoms associated with a check engine light?
When the Check Engine Light comes on, you may experience engine performance issues such as poor acceleration, rough idling, or an engine that won't start. In some cases, no abnormal symptoms will be experienced. Other systems like the transmission or ABS can cause the Check Engine Light to illuminate and lights for those systems can come on at the same time. Similar lights may say "Check Engine Soon", "Malfunction Indicator Light" or just "Check". In rare case the engine can overheat.
Can I drive with the check engine light on?
Whenever the Check Engine Light is on there's a possibility the engine is emitting excessive emissions out the tailpipe. In extreme cases other parts in the system can be damaged, like the Catalytic Converter or Oxygen Sensor. Whether you experience performance problems or not, a flashing CEL indicates that an engine misfire has occurred. When this happens you should pull off the road and not drive the vehicle as this will damage the Catalytic Converter, which can cost over $1000 to replace.
How often does the check engine light come on?
The Check Engine Light can come on at anytime and at any vehicle age. It is not uncommon for this to happen while the car is under warranty and at anytime during the remainder of the vehicle's life.
How is a check engine light diagnosed?
A professional will connect a diagnostic scan tool to read out the diagnostic trouble codes stored in the vehicle. They should check every system in the vehicle as the light can be illuminated from other things like the transmission or ABS system. There can be numerous codes stored in the computer for days, weeks or months so it's important to identify the code that is causing the light to illuminate. The computer can store additional data like when the code was set, how many times it was set, and the sensor reading at the time it was set. All this info is used to start the diagnostic process. Can manufacturers release information called Technical Service Bulletins that may shine a light on why a code was set. Professional mechanics will always check these before diving too deep into any issues that caused the light to come on. Depending on why the code came on, mechanics will test out the sensors or other items like the injectors, fuel pump, spark plugs or just about any other part on the engine. Once a failed part is identified the mechanic replaces the part, resets the trouble codes then drives the car to verify the issue has been resolved.
Related repair information
A Check Engine Light is turned on when the engine control module detects a fault in the emissions system. One or more diagnostic codes will be stored in the engine module. These codes are just one piece of information the shop will use to diagnose the problem. Each code has a diagnostic procedure that must be followed to properly identify the problem.
Other considerations
Some vehicles may display this warning light as the SES (Service Engine Soon) or MIL (Malfunction Indicator Light).
RepairPal recommendations for a check engine light
We recommend that you never replace a part based on only the diagnostic trouble code from the engine computer. Have the system diagnosed by a professional to ensure you don't spend money on parts that aren't needed. When a shop works on your car for this issue ask them to record all the codes stored in the vehicle on the repair order. If they are having trouble diagnosing an issue, ask them if they've checked the Technical Service Bulletins as these can help to identify really hard problems.