The Check Engine Light (CEL), a component of the OBDII (On-Board Diagnostics II) system, is specifically designed to monitor your vehicle’s emissions system. Introduced as a federal mandate in the US in 1996, OBDII standardized diagnostic codes and retrieval methods across all car manufacturers. This standardization allows mechanics and vehicle owners to use generic or specialized OBDII scanners to access diagnostic trouble codes (DTCs) by connecting to the OBDII port, typically located in the driver’s footwell.
The CEL illuminates when the vehicle’s emission control sensors detect a malfunction on two consecutive drive cycles. This second detection triggers the Malfunction Indicator Light (MIL), commonly known as the CEL or Service Engine Soon light.
To diagnose the issue, you can use an OBDII scanner to retrieve the DTCs. Basic scanners will provide a 5-digit code, while more advanced scanners may offer a 6-digit manufacturer-specific code, providing more detailed information about the area of the emissions system where the fault occurred.
Many auto parts stores offer free OBDII code reading services. Once you have the 5-digit DTC, a quick online search can provide initial insights into the potential problem.
It’s important to understand that a lit CEL related to emissions can stem from a wide array of issues. It could be as simple as a loose gas cap preventing proper sealing of the fuel system, or as complex as issues with air-fuel mixture due to faulty sensors or components within the intake or exhaust systems. Essentially, any component affecting the vehicle’s emissions—fuel, ignition, exhaust, and related air systems—can potentially trigger the CEL.
For further clarification, refer to your vehicle’s owner’s manual for specific information regarding the Check Engine Light and the OBDII system. The following image, taken from a BMW owner’s manual, illustrates the general description of the CEL:
