Q: What Is the Problem When the Check Engine Light Comes On?

The Check Engine Light is on most vehicles built in the last twenty years. The light is used to alert the operator of the vehicle that there is a problem that needs to be looked at as soon as possible. The light may mean you have a transmission, engine, suspension, or powertrain problem.

The Check Engine Light is directly connected to the engine control module (ECM). The ECM monitors the inputs and outputs for the engine to run properly in most conditions and keeping the emissions within specifications under all these conditions. When the ECM detects one of the monitored inputs or outputs not within limits that are programmed into the ECM, it will set a code and turn on the Check Engine Light. Some of the most common inputs the ECM monitors is the crankshaft position sensor, camshaft position sensor, coolant sensor, throttle position sensor (TPS), O2 sensors, fuel pressure sensor, exhaust gas recirculation (EGR) valve sensor, mass air flow (MAF) sensor, and intake air temperature (IAT) sensor. Some of the more common outputs the ECM monitors are fuel injectors, spark control to coil or coils, camshaft control solenoids for variable cam timing, intake manifold runner control solenoid, throttle motor actuator on electronic throttle control, EGR position control solenoid, and evaporative vent solenoid. There can be additional inputs and outputs to the ECM that the manufacturers use over the list above.

The ECM on most vehicles is now connected using network cables to the other vehicle modules that control additional vehicle functions like transmission, antilock brakes, suspension, body control, lighting, radio/GPS, or other functions needing a module. Some of these modules may be integrated into a single module and called a powertrain control module (PCM). The PCM may include engine, transmission, and other modules into one module or a vehicle may keep all of them separate and just share information over the network. When any of these other modules have a problem with one of their inputs or outputs, they may have their own failure light that will turn on. Some sensors’ data that the ECM uses from the other modules may affect the way the engine will operate and requires the Check Engine Light to be on. The module having the sensor failure will request the ECM turn on the Check Engine Light and set a failure code in memory.

Some of these failures say, for instance, that the speed sensor is going bad. This sensor is monitored by the anti-lock brake control module for the brakes. The sensor data is shared on the network for all networked computers to use if they need the info. The ECM may need it as a vehicle speed input for fuel control. The transmission control module (TCM) may use it for proper shifting, the body computer module may use it for speed dependent wipers, and the suspension computer module may use it for ride control. There is only one sensor that may have failed, but it causes all the different modules to be affected in some way and multiple functions will not work as designed. The Check Engine Light may only come on by itself or you may get the ABS Light, VSC light, and other lights that require the input of the one sensor to work properly.

When you have the Check Engine Light scanned, the code stored in memory will be displayed on the scanner. The code is designed to direct the mechanic to a particular area of concern and also a particular pinpoint test to follow. The code is not used to swap parts as most people do. It is meant as a tool to help a mechanic properly diagnose the problem. For instance if the Check Engine Light blinks while driving, most commonly it is caused by an engine misfire. When you scan the computer it may give you a code like “P0303”. This code means that cylinder number three had or has a misfiring cylinder. This code does not mean your spark plug failed and replacing the plug will fix the problem. The code simply points the mechanic to an area of concern. The pinpoint test for the code P0303 will take the mechanic through steps. These steps may check the spark plug, coil, coil wire or spark plug wire, wiring to the coil or connector, or an ECM problem. There may be many checks for a mechanic to come to the conclusion that the failure is a leaking valve cover gasket that leaked oil on to the spark plug, shorted the plug wire, and fouled the plug out. The total repair may be replacing the valve cover gasket, ignition coil and coil boot, and the spark plug. Some fixes are not just the face value of the code. Sometimes you may get lucky and replace a part and fix the problem or you may waste your money replacing a part only to have that part fail in a short time due to the root cause of the failure not being fixed.

Remember the Check Engine Light is to alert the operator of the vehicle that there is a problem and should be checked as soon as possible. Don’t delay getting an inspection by a certified mechanic.