GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2008 🢒 Uplander 🢒 Brakes 🢒 Antilock Brake System 🢒 DTC C0121

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC C0121: Valve Relay Circuit

Circuit Description

An internal solenoid valve relay in the electronic brake control module (EBCM) supplies B+ to all solenoid valves. This voltage is referred to as system voltage. The EBCM activates the solenoid valve relay when the ignition is turned ON. Once the solenoid valve relay has been activated, the EBCM supplies ground to actuate individual solenoid valves.

Conditions for Running the DTC

Ignition ON.

Conditions for Setting the DTC

One of the following conditions exists:

    • The EBCM commands the solenoid valve relay on and battery voltage is less than 8 volts at the solenoid valves.
    • An internal EBCM relay driver fault is not detected.

Action Taken When the DTC Sets

If equipped, the following actions occur:

    • The EBCM disables the antilock brake system (ABS)/traction control system (TCS)/vehicle stability enhancement system (VSES) for the duration of the ignition cycle.
    • The ABS indicator illuminates
    • The Red brake indicator illuminates.
    • The Stability Control indicator illuminates.
    • The driver information center (DIC) displays Service Stability control.

Conditions for Clearing the DTC
    • The condition for the DTC is no longer present.
    • The EBCM clears the history DTC when a current DTC is not detected in 100 consecutive drive cycles.

Diagnostic Aids

The solenoid valve relay is integral to the EBCM. The relay is not serviceable.

Reference Information

Schematic Reference

Antilock Brake System Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

ABS Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification
  1. Ignition ON, perform the Solenoid Tests with a scan tool and listen/feel the solenoids clicking.
  2. Verify that DTC C0121 is not set.
    3. ⇒ If DTC is set, replace the EBCM and verify DTC does not reset. If DTC resets replace the BPMV.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for EBCM replacement, setup, and programming