High Voltage Battery Disconnect Relay Fault

Common Symptoms

• Malfunction Indicator Lamp (MIL) illuminates on the instrument cluster
• Vehicle may display a 'Service High Voltage Charging System' or 'Propulsion Power is Reduced' message
• Reduced acceleration or limited driving range noticed after a recent battery software update or pack repair
• Vehicle enters a reduced power or limp mode, limiting top speed to approximately 55 mph
• Charging may be interrupted or refused at Level 2 or DC Fast Charge stations
• P0AA1 appears in HPCM2 stored or history DTCs when scanned with GDS2 or Autel MaxiSys
• No visible warning before the fault, but code surfaces during routine post-repair scan

Probable Causes (Ranked by Likelihood)

• High-voltage battery disconnect relay degraded or failed due to excessive inrush current during pack reconnection after software update or repair Very Likely
• Inrush current spike during battery pack reconnection caused relay contact welding or internal damage Very Likely
• BECM or HPCM2 software version mismatch following a GM software update that altered relay pre-charge sequencing Likely
• Incomplete or improper high-voltage battery service disconnect procedure performed during prior repair, stressing the relay Likely
• Corroded or loose high-voltage harness connector at the battery disconnect relay causing intermittent resistance spikes Possible
• BECM internal fault incorrectly commanding disconnect relay open/close sequence out of timing Possible
• Pre-existing relay wear in higher-mileage packs, with the software update triggering a threshold detection that was previously borderline Less Likely

Step-by-Step Diagnostic Procedure

1. Connect GDS2 or a compatible scan tool such as Autel MaxiSys or Foxwell NT530 with GM software to the OBD port. Generic OBD-II readers will not read HPCM2 or BECM module data. Confirm P0AA1 is present in HPCM2 history or current DTCs before proceeding.

2. Check for any related DTCs stored alongside P0AA1, especially P0AA6, P0AC0, P1E00, or U3xxx CAN communication codes. Multiple codes at once can point toward a BECM fault rather than the relay alone.

3. Using GDS2, navigate to HPCM2 module data and review the 'HV Battery Disconnect Relay' parameter. Note whether the relay command state and actual state agree. A mismatch confirms relay hardware failure.

4. Inspect the high-voltage orange service disconnect plug (located under the rear seat or in the rear cargo area depending on model year) for proper seating. A partially seated disconnect can mimic relay faults.

5. With the vehicle powered down and the high-voltage service disconnect removed, visually inspect the high-voltage harness connectors at the battery disconnect relay for corrosion, backed-out pins, or heat damage. Do not probe high-voltage circuits without proper high-voltage PPE and training. Stop here and call a qualified EV technician if you are not HV-certified.

6. Check for any pending GM software calibrations for HPCM2 or BECM using GDS2. If a calibration update is available, install it and re-run the diagnostic to see if the code re-sets. GM has issued software updates addressing relay pre-charge timing on affected Bolt packs.

7. If GDS2 confirms the relay command and actual state are mismatched and no software update resolves the fault, GM service guidance tied to NHTSA MC-10214720-9999 directs replacement of the high-voltage battery disconnect relay assembly. This step requires HV-certified technician hands-on work.

8. After relay replacement, clear all DTCs with GDS2, perform a complete ignition cycle, and re-scan all modules to confirm no codes return before returning the vehicle to service.

Frequently Asked Questions