What to Do About the Check Engine Light

Since Karl Benz paired the internal combustion engine with wheels, engines and cars have become ever more powerful and efficient. Thanks to the addition of advanced sensing and fast-acting computer controls, today’s engines are better than ever. Whereas auto diagnostics used to be based on specific concerns, drivers often have no idea what the problem is when their vehicle’s check engine light (CEL), also known as the service engine soon (SES) light or malfunction indicator lamp (MIL), comes on. How does it work?

With the addition of electronic controls, such as fuel injection and emissions monitoring systems, came the need for advanced diagnostic strategies. Since 1996, on-board diagnostic (OBD) systems have been built into every vehicle on the road, enabling various controllers called electronic control units (ECU) to detect faults. Today’s vehicles have many ECUs, including for the engine, transmission, brakes, anti-lock brakes, air conditioning, and lane departure warning system.

What Is the Check Engine Light?

Depending on the fault, the check engine light may be the only indication of a problem. The engine control module (ECM), for example, uses sensors to monitor coolant temperature, oil pressure, exhaust oxygen content, and other aspects of engine operation. The ECM tests sensors to see if they seem to be reacting properly. If the ECM detects a problem, it illuminates the MIL and saves a diagnostic trouble code (DTC) in memory; it may also save freeze frame data, namely, sensor readings at the time of the fault.

Some systems are disabled if there is a fault, such as if the lane departure warning system detects a fault with the camera. In the case of the engine, rather than being disabled in the event of an oxygen sensor fault, it goes into open loop mode, also called limp home, limp in, or fuel backup mode. This backup mode gives the driver basic engine operation, but possibly with reduced performance, increased emissions, or reduced fuel economy. If you’re out on a summer road trip, you might spend more on fuel or lag going up hills.

How Do You Fix a Check Engine Light?

If the CEL comes on, even if your vehicle seems to be running well, the issue needs to be diagnosed and repaired as soon as possible. Timing is critical, as the CEL indicates that current running conditions could damage the catalytic converters—a very expensive repair. Fixing the CEL isn’t about fixing the light itself, but the problem causing it to illuminate.

First, you need to connect an OBD scan tool or code reader to your vehicle. The scan tool communicates with the ECM, accessing information such as sensor readings and DTCs (of which there are roughly 5,000). The DTC indicates only what the ECM is detecting, not the exact problem. The component, its circuit, and other parts will be diagnosed by comparing them with accepted specifications. Once the problem is found, adjustment, repair, or replacement will restore proper function. Finally, the scan tool can be used to erase the DTCs and verify the repair, returning the electronic controller to normal operation.

If the CEL comes on, the problem may be as simple as worn spark plugs or as complex as an electrical fault in the ignition system. Getting a proper diagnosis and repair will get your car back in top condition. Check out the check engine light services available at our 600 NAPA AUTOPRO locations for routine maintenance and repairs. For more information on your check engine light, chat with an expert at your local NAPA AUTOPRO service centre.