How to Diagnose a Bad PCM in a Dodge Cummins

by Richard Ristow
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Car dashboard image by Andrejs Pidjass from

A powertrain control module (PCM) monitors functions and sensors within a Dodge pick-up's Cummins diesel engine. Once the PCM locates a problem, it assigns an error code. If the problem persists, the PCM turns on the Dodge's "check engine" light. This is all part of the Environmental Protection Agency's standardization of On-Board Diagnostics. The system also has self-diagnostic capabilities: Once a Dodge's PCM ceases to work, a trouble code is generated. Checking for PCM malfunctions is similar to checking the OBD-II system: You need a code reader or a diagnostic scanner.

Step 1

Research OBD-II codes -- you will need to find two sets: Generic OBD-II codes are universal to all post-1996 vehicles, and they are usually found in your code reader or scanner's user's manual. Chrysler uses an additional set, one that is particular to all vehicles in the Chrysler family. You will need to locate these codes either online or in a Haynes repair manual.

Step 2

Read through both sets of trouble codes: Make a list of all the codes that deal with the PCM. For example, code P1602 refers to the PCM not being programmed. P1696 and P1670 deal with particular types of computer failure within the PCM. Place this list of codes in the Dodge's center console.

Step 3

Connect your code reader or diagnostic scanner to the Dodge's Data Link Connector. The DLC outlet will be positioned differently in different models that use a Cummins engine. However, it will usually be found within the driver's side leg space: either beneath, to the left or to the right of the steering column.

Step 4

Turn either the code reader or the scanner and the Dodge's electrical "on." Some brands and types of diagnostic hardware will also need the engine to be idling. If you own such a device, start the Dodge's engine at this time.

Step 5

Look at the code reader or scanner's small display screen. If you do not see diagnostic codes displayed, you will have to retrieve them. Retrieval commands differ according to the brand and type of device. Buttons are located in different positions. Consult your device's manual for the exact instructions to follow.

Step 6

Scroll through the codes retrieved. Since you are looking for PCM functionality, you can ignore any code listed as "pending." Consult your list of PCM-related trouble codes. If the code reader or scanner did not retrieve PCM-related codes, then the component is working and is not in need of troubleshooting. If you do see PCM-related codes, then the module itself may need to be reprogrammed or replaced. You may need to consult a Chrysler-approved mechanic. PCM modules are usually not "off the shelf" merchandise, and you will need to order one that meets your Dodge's exact needs.

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