How to Troubleshoot the PCMby Richard Ristow
A Powertrain Control Module (PCM) is a car's central diagnostic computer. It monitors a vehicle's engine and fueling system, and once a problem is detected, the PCM turns on the vehicle's "Check Engine" light. This light will also activate if the PCM begins to falter or become unresponsive. While you can locate the module and check the wiring, the easiest way to troubleshoot this component is by using an On-Board Diagnostic (OBD-II) scanner. The module has self-diagnostic codes programmed into it.
Locate your vehicle's PCM. The position of the module may differ slightly according to the make and model of the vehicle. For example, on some vehicles the PCM is toward the back of the engine compartment. On other vehicles, it is under the dashboard on the passenger's side.
Look specifically at the wiring harness. If it is detached or frayed, the PCM will not be able to communicate with the sensors throughout the car. This alone will turn on a "Check Engine" light. If the harness is detached, reattach it and check to see if the "Check Engine" light goes off. If it does not, the harness may need to be replaced. However, it's best to try and pull the trouble codes from the PCM first.
Locate your vehicle's diagnostic data port. This outlet is usually beneath the dashboard on the driver's side of the vehicle. This port is in plain sight on many vehicles. In others, however, it is concealed behind a panel.
Connect your OBD-II scanner to the diagnostic data port.
Push your key into the vehicle's ignition cylinder. Turn the key to "On." This will turn on the electrical system. Some brands of handheld devices may require you to start the engine. Turn on the engine if you own this type of scanner.
Watch the OBD-II device's read-out screen for the codes. If no code appears, ensure the device is on and enter a "scan" command. The process for this will be slightly different by scanner brand due to differing button configurations. Defer to the exact instructions offered in the device's manual.
Look up each code and temporarily skip the ones that do not apply.
Decide whether the PCM problems can be addressed without a mechanic. Some issues may only require restarting the PCM which can be accomplished by safely disconnecting the vehicle's negative battery cable, waiting for the power to drain and reconnecting the cable. Refer to a "Haynes" or "Chilton" manual for assistance.
- "Haynes" and "Chilton" manuals for your make and model can be purchased online, new or used. In some instances, they may be available at your local public library.
- Some vehicle issues may be more complex and should be deferred to a mechanic.
- PCM modules are not readily off-the-shelf parts and may need to be specially ordered.
- A bad PCM needs to be fixed or replaced. If your vehicle is up for a state inspection, an active "Check Engine" light will cause it to fail.
Items you will need
- OBD-II scanner
- "Popular Mechanics Complete Car Care Manual" The Editors of Popular Mechanics; Hearst Publishing; 2008
- US EPA: OBD Basics