GM Service Manual Online
For 1990-2009 cars only

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 Descriptors

DTC P0AFA: Hybrid Battery System Voltage Low

Circuit/System Description

The starter generator control module (SGCM) monitors the hybrid battery voltage to verify that the output voltage (36 volts) Is within normal operating value (above 22 volts). If the output voltage is not within this value, this DTC will set.

Conditions for Running the DTC

The engine is running.

Conditions for Setting the DTC

36 volt output voltage is less than 22 volts for 2 seconds.

Action Taken When the DTC Sets

    • Stop PWM (MGU shutdown)
    • APM PWM off

Conditions for Clearing the DTC

36 volt output voltage increases to greater than 22 volts.

Diagnostic Aids

    • Possible battery cable open
    • Possible fuse/relay block electrical failure

Reference Information

Schematic Reference

    •  Hybrid Controls Schematics
    •  Hybrid Energy Storage Schematics

Connector End View Reference

    •  Hybrid Controls Connector End Views
    •  Hybrid Energy Storage Connector End Views

Electrical Information Reference

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

Scan Tool Reference

Scan Tool Data List

Circuit/System Verification

  1. Ignition ON, observe the scan tool Hybrid System Voltage parameter in the SGCM data list. The voltage should be greater than 22 volts. If the parameter displays less than the specified value, refer to SGCM 36 volt Output Terminal Circuit Low.
  2. Ignition ON, observe the scan tool Hybrid System Voltage parameter in the BECM data list. The voltage should be greater than 22 volts. If the parameter displays less than the specified value, refer to Battery Pack Voltage Low.

Circuit/System Testing

Caution: To help avoid personal injury, additional precautions must be taken prior to working on the generator control module or the generator starter. After removing the 36V battery cables from the generator battery, remove both engine wiring harness connectors from the generator control module. Wait at least 5 minutes and then remove the generator control module cover. Verify voltage levels at all 36V, 12V, and 3-phase connections, are less than 3 volts using a DMM before proceeding.

Caution: To help avoid personal injury, be careful when working in the vicinity of the generator battery disconnect control module. Internal components will still be live, 36V potential, even when the cover has been opened or removed.

SGCM 36 volt Output Terminal Circuit Low

  1. If DTC is history, refer to Testing for Intermittent Conditions and Poor Connections .
  2. Ignition ON, test for greater than 22 volts at the SGCM battery positive 36 volt output voltage stud.
  3. If less than the specified value, test the 36 volt cable for an open/high resistance or short to ground.
    If greater than the specified value, replace the SGCM.

Hybrid Battery Pack Voltage Low

  1. If DTC is history, refer to Testing for Intermittent Conditions and Poor Connections .
  2. Gain access to the hybrid battery pack. Refer to Hybrid Battery Service Disconnect/Connect.
  3. Ignition OFF, test battery pack voltage for greater than 22 volts.
  4. If less than the specified value, replace the battery pack.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Control Module References for SGCM replacement, setup, and programming
    •  Generator Battery Replacement