Diagnostic article Generic OBD-II
P0AF8: Battery and generator-controller voltage measurements disagree
P0AF8 compares hybrid battery voltage with the starter-generator controller output voltage in this GM system.
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Documented application
2009 General Motors diagnostic parameter file 09OBDG08 Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P0AF8 is a voltage-correlation check in GM's BAS hybrid summary. It compares the energy-storage controller's battery voltage with the starter/generator controller's output-voltage information. The documented difference threshold is greater than five volts for four seconds.
The connection state qualifies the comparison
The main relay must be closed, the supplied battery-voltage information must be valid and the listed pre-flux current condition must be met. Those prerequisites matter: two voltages taken while the system is intentionally disconnected do not necessarily reproduce this diagnostic test.
The source states that the comparison is intended to detect connection problems involving the phase-cable, fuse or relay path. It does not establish which element failed from P0AF8 alone. Measurement validity remains part of the interpretation.
Correlation differs from low system voltage
The neighboring P0AFA monitor evaluates output below a fixed low-voltage limit. P0AF8 instead evaluates disagreement between two voltage views. Both measurements could be above a low-voltage threshold yet still fail their correlation.
Preserve both controller readings, connection state and companion faults. Use the exact hybrid diagnostic and isolation procedures through qualified service; do not bridge fuses or relays to test the theory. Verification should show credible agreement with the required connection state and completed monitoring. An erased code or a normal reading from only one controller cannot establish that the original voltage disagreement is resolved.
Source and applicability
2009 General Motors diagnostic parameter file 09OBDG08 Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established. This explanation describes the documented monitor, not a universal repair procedure.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Hybrid/EV Battery System Voltage
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT