GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2009 🢒 Yukon XL - 2WD 🢒 Driver Information and Entertainment 🢒 Cellular, Entertainment, and Navigation 🢒 DTC P1A21

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 P1A21: Hybrid Battery Contactor Control Sequence Incorrect

Circuit/System Description

The drive motor generator battery control module is also referred to as the battery energy control module (BECM). The BECM will diagnose its own systems and determine when a fault condition is present. Diagnostics and system status is communicated from the BECM to the hybrid powertrain control module (HPCM) through the Hybrid LAN communication circuit. The HPCM is the host controller for diagnostic trouble code (DTC) information.

The HPCM controls the operation of the high voltage (HV) contactor relays through communication with the BECM. The HPCM utilizes a pulse width modulated (PWM) contactor system request circuit and a redundant GM LAN message to command the HV contactor relays Open and Closed. Under normal operating conditions, the BECM opens and closes the HV contactor relays based only upon the HPCM command. This diagnostic identifies when the BECM has opened the HV contactor relays when the HPCM is commanding them closed.

Conditions for Running the DTC
    • The vehicle power mode is RUN.
    • Accessory voltage is 9-18 volts.
    • The vehicle has passed a successful precharge and the High Voltage contactor relays have been closed this ignition.
    • The HPCM is commanding the BECM to keep the HV contactor relays closed.

Conditions for Setting the DTC

The BECM reports that the contactor relays are open when the contactor system request circuit and the GMLAN message both indicate a closed request.

Action Taken When the DTC Sets

DTC P1A21 is a Type A DTC.

Conditions for Clearing the DTC

DTC P1A21 is a Type A DTC.

Diagnostic Aids
    • The BECM will set contactor status to OPEN when the percent duty cycle is 65-75 percent.
    • The BECM will set contactor status to CLOSED when the percent duty cycle is 25-35 percent.
    •  The frequency of the PWM signal must be between 247 Hz to 253 Hz to be valid.

Reference Information

Schematic Reference

Hybrid Energy Storage Schematics

Connector End View Reference

Component Connector End Views

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, observe the hybrid battery contactor system request circuit commanded parameter using a scan tool. The contactor system request circuit commanded should be 30 percent.
  2. Ignition ON, observe the DTC information with a scan tool. DTC P1A21 should not set.
  3. Operate the vehicle within the Conditions for Running the DTCs. You may also operate the vehicle within the conditions that you observed from the Freeze Fame/Failure Records data.

Circuit/System Testing

Verify that DTCs P1AE4, P0A80, P1ABE, or P1A0C are not set.

⇒If any of the listed DTCs are set, repair them first. Refer to Diagnostic Trouble Code (DTC) List - Vehicle.
⇒If any of the listed DTCs are not set, replace the drive motor generator battery control module.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

Control Module References for drive motor generator battery control module replacement, setup and programming.