Diagnostic Test

WARNING: This page is about a different car, the 2009 Jeep Grand Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

TEST FOR INTERMITTENT CONDITION
1. Turn the ignition on.
2. With the scan tool, record and erase FCM DTCs.
3. Cycle the ignition from on to off 3 times.
4. Turn the ignition on.
5. With the scan tool, read active FCM DTCs.
  Does the scan tool display this DTC as active?
  
  Yes
  - Go to step 2
  No
  - The conditions that caused this code to set are not present at this time. Using the SYSTEM WIRING DIAGRAMS as a guide, inspect the wiring and connectors.
CHECK THE (D55) CAN B BUS (+) CIRCUIT FOR A SHORT TO THE (D54) CAN B BUS (-) CIRCUIT

Fig 1: Checking CAN B BUS (+) Circuit For A Short To CAN B BUS (-) Circuit
Courtesy of CHRYSLER LLC
1. Turn the ignition off.
2. Disconnect the Front Control Module C1 harness connector.
3. Measure the resistance between the (D55) CAN B Bus (+) circuit and (D54) CAN B Bus (-) circuit.
  Is resistance below 1000.0 ohms?
  
  Yes
  - Go to step 3
  No
  - Inspect the wiring and connectors for damage or shorted circuits. If ok, replace the Front Control Module in accordance with the Service Information.
  - Perform the BODY VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Front Control (FCM) - Standard Procedure
(D55) CAN B BUS (+) CIRCUIT SHORTED TO THE (D54) CAN B BUS (-) CIRCUIT

Fig 2: Checking CAN B BUS (+) Circuit For A Short To CAN B BUS (-) Circuit
Courtesy of CHRYSLER LLC
1. Measure the resistance between the (D55) CAN B Bus (+) circuit and (D54) CAN B Bus (-) circuit.
2. While monitoring the ohmmeter, disconnect each CAN B Bus module one at a time.
  NOTE: This is to determine if the short together is internal within a module or if the circuits are shorted together.
  
  NOTE: Disconnecting an in-line connector can eliminate a module or group of modules from the list of possible causes for this fault. Refer to the SYSTEM WIRING DIAGRAMS to assist in diagnosis.
  
  Is resistance below 1000.0 ohms with all the CAN B Bus modules disconnected?
  
  Yes
  - Repair the (D55) CAN B Bus (+) circuit for a short to the (D54) CAN B Bus (-) circuit.
  - Perform the BODY VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Front Control (FCM) - Standard Procedure
  No
  - Replace the module that when disconnected the short together was eliminated, in accordance with the Service Information.
  - Perform the BODY VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Front Control (FCM) - Standard Procedure