Diagnostic Test

WARNING: This page is about a different car, the 2010 Dodge Charger and 2010 Chrysler 300. However, it is still accessible from the selected car via links, so may be relevant.

CHECK FOR A DTC C121D-BRAKE PRESSURE SENSOR CIRCUIT
NOTE: This DTC must be active for the results of this test to be valid.
1. Turn the ignition on.
2. With the scan tool, read and record DTCs.
3. With the scan tool, read and record Environmental Data to help identify the conditions in which the DTC was set.
4. With the scan tool, erase DTCs.
5. Start the engine.
6. Slowly depress and release the brake pedal.
7. With the scan tool, read and record DTCs.
  Does the scan tool display: C121D-BRAKE PRESSURE SENSOR CIRCUIT?
  
  Yes
  - Go to step 2
  No
  - Perform the INTERMITTENT CONDITION diagnostic procedure. See DTC-Based Diagnostics/MODULE, Antilock Brake (ABS) - Standard Procedure .
HCU/ECU ISOLATION
1. Turn the ignition off.
2. Separate the ECU from the HCU.
3. With the scan tool, monitor brake pressure.
  Does the scan tool display brake pressure above 218 bar (3162 psi)?
  
  Yes
  - Replace the HCU in accordance with the Service Information.
  - Perform ABS VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Antilock Brake (ABS) - Standard Procedure .
  No
  - Replace the ECU in accordance with the Service Information.
  - Perform ABS VERIFICATION TEST. See DTC-Based Diagnostics/MODULE, Antilock Brake (ABS) - Standard Procedure .