Car Repair-Computer Systems

Today’s cars all have computers, otherwise known as the PCM (Powertrain Control Module) or ECM (Electronic Control Module). (For purposes of this article, we will use PCM). The PCM has many different functions – regardless of which type of car you are driving, they all have the same function.

The powertrain control module is the component that turns your “check engine” light on. It will turn the light on if one of the sensors is bad. It will also turn the light on if there is something mechanical wrong with the vehicle, and the mechanical error affects the reading of one of the sensors. These are normally known as “soft codes.”

Certain sensors provide input to the powertrain control module. Other sensors provide output so that the car can run smoothly and efficiently. If a sensor goes bad, it will give a “hard code” and turn the check engine light on in the dash.

Like the computer components in your home computer, it can also be shorted out with something as small as a static shock from your fingertips. When jumpstarting your car, be sure to attach the cables while both vehicles are turned off, and the keys to both vehicles are in the off position.

Certain years of certain makes and models (most notably early to mid 90’s Toyotas and Hondas) had problems with the distributor shorting out. This creates a diagnostic nightmare, as there is no engine light. We have found that approximately 40 percent of the time, the distributor shorted out, flashed the powertrain control module with too much “juice” and shorted out the powertrain control module.

The powertrain control module does go bad on some cars. The mileage varies – some as early as 120,000 miles, some never going bad. There is no real way to test the powertrain control module. It cannot diagnose itself, so scanners such as Matco’s OTC4000 or SnapOn’s Vantage will not tell you if a computer is bad.

There are a couple of “hints” the powertrain control module will give you to tell you the problem lies with the powertrain control module. If you attach a scanner and get erroneous codes, chances are, the powertrain control module has “burned up.” Each car has its own set of codes. A scanner translates these codes. Also, if the car is not running, and you pull the powertrain control module, smell the plugins. If the powertrain control module smells “fishy,” chances are it’s bad. Another check would be to put heat or cold to it during diagnosis (please note, that some powertrain control modules on certain year vehicles will not withstand this test). For intermittent problems with no telltale engine light or erroneous codes, sometimes you can recreate the problem by using a hair dryer to heat up the computer to see if the vehicle will shut off.

A soft code will turn the check engine light on, but it may not stay on. A hard code must be erased via the computer. On certain older years, removing the negative battery cable for approximately five (5) minutes can erase codes.

An example of a soft code is the oxygen sensor code. If your vehicle needs a tune up, and the plugs are not burning properly, your vehicle can run rich (too much gas) or lean (too much air). Most cars run on a 14:1 air/fuel mixture. If this is off, the oxygen sensor (which is located in the exhaust pipe) will sense this and turn the check engine light on. If the problem clears on its own (i.e. if you “blow it out,” meaning you run the engine at high speeds – not recommended unless you are at the race track!), the plugs will be cleaned off and the light will go out. You do not need to erase this code. You may also change the spark plugs, and the code will erase itself.

Your oxygen sensor would send a hard code, making the check engine light stay on if the sensor itself or the wiring to the sensor is bad. Any sensor that is bad will send a hard code.

A lot of powertrain control modules have a “limp home mode.” If your car starts, it may not run right, but may get you a short distance to the mechanic or home.

If your check engine light comes on, even if the vehicle seems like it is running smoothly, take it to your mechanic immediately, and have him scan the codes. He will be able to advise you as to the problem.