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

asked by on December 01, 2015

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.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

What others are asking

My check engine light is on

A vehicle that has a problem with the engine or power train can turn on the check engine light and set a code in the computer. The list of possible codes and problems are in the hundreds. You will need...

Car shaking when applying brakes and going slow. Check Engine Light is on and engine idles roughly.

It sounds like you may have a vacuum leak on your car. Check all vacuum lines and make sure they are in good condition and connected. If you don't see a leak you can spray some carburetor cleaner on the...

RPM/engine issue.

The engine's computer should be scanned first to see if there are codes pointing to the cause of the RPM problems and the no start. If the codes are tested and no problems are found then try and disconnect the...

P0328 code - 2008 Hyundai Accent

Hello. P0328 is the code for a knock sensor. If the knock sensor fails, or there is an issue with the wiring, the code may be set off. You also mention that the Check Engine Light was not on. If...

My check engine light came on...car isn't overheating no change in engine temp gauges..autozone used one of their testing gages sa

There are several possible reasons for a check engine light to come on. Typically, when the check engine light appears, the first step is to pull the diagnostic codes out of the computer with a scan tool, like you did....

Engine died, do I have to replace it now?

The noise you are hearing could be one of several problems. The first and easiest to fix would be to use only manufacturer certified oil filters during oil and filter changes. The oil filter used may not have a built-in...

Coolant finishes 3-4days

Hello. This can be caused by a number of things such as low coolant levels, a faulty thermostat, or a failing coolant fan switch. The fact that you are losing water every 3-4 days without leaks, this would suggest the...

Check engine light on

The code P0442 is for a very small leak in the evaporative system of the fuel tank and its vent system. A leak in the gas cap is the most common cause of this code and the gas cap or...

Which sensors are most important for monitoring and improving the engine’s performance?

The way modern electronic engine control systems are designed, all of the sensors are important but some are more crucial to the vehicle than others. For example, if the engine electronic control unit (ECU) loses the input from the coolant...

Related articles

P2265 OBD-II Trouble Code: Water in Fuel Sensor Circuit Range/Performance
P2265 P2265 code definition The P2265 diagnostic trouble code is an indication that there is a range/performance issue with the water in fuel sensor circuit. Related Trouble Codes: P2264 OBD-II Trouble Code: Water in Fuel Sensor Circuit P2266 OBD-II...
P0144 OBD-II Trouble Code: O2 Sensor Circuit High Voltage (Bank 1, Sensor 3)
P0144 P0144 code definition O2 Sensor Circuit High Voltage (Bank 1 Sensor 3) What the P0144 code means P0144 is the OBD-II generic code which indicates that the Engine Control Module (ECM) has detected excessively high voltage from the bank...
P2100 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit Open (TACM)
P2100 code definition Throttle Actuator Control Motor Circuit Open (TACM) What the P2100 code means P2100 is an OBD-II generic code which...