There Goes that Check Engine Light Again

Is your check engine light on? The check engine light on your instrument panel is in the shape of an engine, is yellow and usually has the words check engine on it. This light indicates a problem within the computer's network of influence and has nothing to do with the mechanical aspect of the engine or drive train.

The computer is called the PCM or ECM. Its function is to control the drivability and the exhaust emissions of the vehicle. It has nothing to do with the oil level or internal problems with the engine. In essence, you do not have to shut the engine off when this light comes on. There are lights on the instrument panel that are designed to indicate low oil pressure such as a red light and usually says oil. The same applies for the engine temperature.

If you picture a rectangular box with at least 10 wires extending from either side - on one side the wires are input from the sensors. The computer takes the input from all these sensors and makes analytical calculations many times a second. It then operates, thru the second set of wires, the devices that operate the engine. Actually, the computer is generally a box 1 ½ to 2 inches thick and about 6 to 8 inches square or rectangular and has three to four harness connectors plugged into it. It also has a 2-inch (or so) square plate on the top of it that runs the close to the length of it and is used for access to the PROM, or the programmable memory.

The reason that I am describing it is for the fact that modern day cars have many computers and modules for various functions. Here we are only concerned with the ECM.

When the check engine light illuminates, you can do one of two things. Take the vehicle to a service station so that they can diagnose the problem or diagnose it yourself and at least save money by knowing what is wrong.

The computer stores (in its memory) any malfunctions that may have occurred, and in many cases how often the malfunction occurred, as well as the part that is not operating properly. This information can be accessed thru an ALDL port which is a connector usually on the driver's side under or close to the steering column.

To access this information you will need to have a code scanner. These can be purchased at any auto parts store and are not expensive. The computer gives off a code describing the problem in a series of numbers. The code scanner will come with a list of the codes and their relative description.

To use the scanner, first locate the connector (ALDL). Plug in the scanner and turn the key on, but do not start the car. Make sure the key is in the rum position and the dash lights are on. The scanner screen will say "scan." Press scan and wait for the code. It will come up with a number such as P1145 for example. This is the area or part that has either malfunctioned or sent a signal to the computer that was not in the normal parameter for proper operation.

Now this is very important so burn this into your mind - there are hard codes (the light stays on) and soft codes (the light comes on and goes off). When you retrieve these codes don't take it to the bank that it indicates a definite failure of the part. Other factors can cause the part to give the wrong signal. For example, an oxygen sensor is one of the few sensors that can bring the light on and off (give a soft code). This codes means that the sensor is either malfunctioning or at the time the light came on it sensed unburned gas in the exhaust. This could be due to a bad spark plug, wire or bad gas among other things. For whichever part the code said is malfunctioning, check the electrical connection first. If it is okay, then test the part.

Here is a list and brief description of the sensors you are concerned with:

Throttle position sensor - this is a potentiometer and is always on the opposite side of the throttle plate, It senses how much you are stepping on the gas.

Coolant temperature sensor - senses the temperature of the engine to regulate the fuel mixture.

Mass Airflow sensor - senses how much air is entering the engine for fuel flow.

Intake air temperature sensor - senses what the air temperature is.

Map sensor - measures atmospheric pressure.

EGR sensor - measures position of sensor, how far open or closed.

Knock sensor - senses resignation, carbon build up or anything that causes a knock in the engine that could be harmful to the engine.

Oxygen sensor - senses exhaust for oxygen content and unburned fuel.

Speed sensor - senses vehicle speed.

Yaw rate sensor - senses to much lean in a turn caused by going to fast in a turn for proper control of vehicle so the ABS computer starts to apply the brakes on one side of the car on the inside of the turn.

Steering angle sensor

Crankshaft sensor - senses top dead center on crank for ignition timing.

Camshaft sensor - used for fuel injection synchronizing and timing.

This is a good list of the sensors to be concerned with. Since the introduction of fuel injection on cars, it has remained relatively unchanged with the exception of a few additions. Learn how to diagnose one car and you can diagnose any car.

With a book on your car from an auto parts store you can diagnose many problems. If nothing else, the purchase of a scanner affords the opportunity to find the problem and call the auto parts store and get a price before you go into have it serviced. You have the advantage of knowing what the repair should cost. This could save you a lot of money.