GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2006 🢒 GTO 🢒 DTC 🢒 DTC C0121

Circuit Description

The electronic brake control module (EBCM) energizes the solenoid valve relay when the ignition switch is ON. The solenoid valve relay applies voltage to the solenoid valve coils. The solenoid valve relay remains energized until the ignition switch is turned OFF, or the ABS/TCS is disabled.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC C0121 Valve Relay Circuit

Conditions for Running the DTC

DTC C0121 can be set anytime the ignition switch is in the ON position.

Conditions for Setting the DTC

The EBCM detects a short, low voltage, invalid signal, or incorrect valve relay operation.

Action Taken When the DTC Sets
    • The ABS and traction control system are disabled.
    • The ABS, TRAC OFF and BRAKE warning indicators turn ON.

Conditions for Clearing the DTC

The condition responsible for setting the DTC no longer exists and the scan tool Clear DTCs function is used.

Diagnostic Aids

Thoroughly inspect the wiring and the connectors. An incomplete inspection of the wiring and the connectors may result in a misdiagnosis, causing a part replacement with the reappearance of the malfunction.

The following are possible causes of the malfunction:

    • An open, or high resistance in the EBCM ground circuit
    • An open, high resistance or short to ground in the EBCM battery supply voltage circuits
    • An internal EBCM malfunction

If an intermittent malfunction exists, refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests the EBCM ground circuit for an open, or high resistance.

    2.

  2. This step tests the battery supply voltage circuits of the EBCM for an open, high resistance or short to ground.

Step

Action

Yes

No

Schematic Reference: Antilock Brake System Schematics

Connector End View Reference: Antilock Brake System Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Use the scan tool in order to clear the DTCs.
  4. Using the scan tool, perform the hydraulic modulator solenoid valve test in the special functions menu.

Did the test pass?

Go to Step 3

Go to Step 4

3
  1. Install a scan tool.
  2. Use the scan tool in order to clear the DTCs.
  3. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Did DTC C0121 reset?

Go to Step 4

Go to Diagnostic Aids

4

Is DTC C0800 also set?

Go to DTC C0800

Go to Step 5

5

Test the electronic brake control module (EBCM) ground circuit for an open, or high resistance. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 6

6

Test the EBCM battery supply voltage circuits for an open, high resistance or short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Step 7

7

Inspect for poor connections at the harness connector for the EBCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 9

Go to Step 8

8

Replace the brake pressure modulator valve (BPMV) assembly. Refer to Brake Pressure Modulator Valve Replacement .

Did you complete the replacement?

Go to Step 9

--

9
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK