How to Test an Automobile Computer

by Richard Ristow
itstillruns article image
Speedometer image by Sirena Designs from

An automobile's central computer is typically identified as a powertrain control module, electronic control module or electronic control unit. Testing any version of your car's computer requires the same diagnostic procedures used for pulling trouble codes from the vehicle's On-Board Diagnostic system.

Step 1

Compile a resource list before running diagnostics on your automobile. Look for two groups of OBD-II codes. Your OBD-II scanner's operations manual will feature an appendix or chapter on trouble codes; it lists generic trouble codes used by all post-1996 automobiles.

Step 2

Locate a second set of OBD-II codes. These codes are determined by your vehicle's brand. Each manufacturer uses a separate set of supplemental codes. Some manufacturers are grouped together into corporate families. For example, General Motors encompasses Chevrolets, Buicks, and Oldsmobile. Chrysler covers Jeep, Dodge, and Chrysler brands. Find these codes online (See Resources).

Step 3

Open the driver's side door. Place your code resources on top of the dashboard. Find the 16-prong receiving-plug outlet located in the leg-space area This port is called a Data Link Connection; its location will differ by brand, model, and year. It is typically uncovered, and to the lower left or the right of the steering column. DLC locators are available online (See Resources).

Step 4

Connect your OBD-II scanner to its diagnostic cable. Insert the cable's 16-prong plug into the DLC outlet. Switch the scanner on, if it is not a brand that features auto-activation.

Step 5

Put your key into your automobile's ignition and turn to the "On" position. Leave the engine off. This brings up your automobile's electrical system, and powers the vehicle's PCM, ECM or ECU. For some OBD-II devices, the electrical system may not be enough. Some handheld scanners require a running engine.

Step 6

Look at your scanner's display screen. Check that your device is interfaced with the automobile's PCM, ECM or ECU. Consult your manual, and follow the exact instructions on how to enter a "scan" command. The procedure will vary by device.

Step 7

Look at your scanner's display, and scroll through the alpha-numeric codes. Write down all the codes that are classified as "trouble." Your scanner's manual explains the difference between "trouble" and "pending" codes. Not all of these codes deal with your automobile's computer.

Step 8

Retrieve your coding resources off the top of the dashboard. Look up all the relevant coding definitions; copy them next to the alpha-numeric numbers on your list. Place asterisks next to everything computer related.

Step 9

Take your automobile to a mechanic if you verify any problems with your vehicle's computer. PCM, ECM or ECU modules are one complete unit. Fixing or repairing usually consists of replacing, restarting or reprogramming them. The computer module may be obsolete on older vehicles.

More Articles

article divider