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DTC codes / P0Axx · Hybrid propulsion / P0A29

P0A29Battery Power Off Circuit High

P0A29 indicates that the hybrid vehicle's battery power-off circuit has been detected as having an abnormally high voltage or an open circuit when the system expects it to be grounded or at a low reference level. The computer sets this code when it sees the control line for the high-voltage battery contactor or power-off relay stuck high, which can prevent the high-voltage system from safely shutting down or powering up.

SystemPowertrain
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaHybrid propulsion

Symptoms you may notice

  • Often no symptoms besides the warning light, especially if the fault is intermittent.
  • The hybrid system may fail to start or enter 'Ready' mode, leaving the vehicle unable to move.
  • A message such as 'Check Hybrid System' or 'Service Hybrid System' appears on the dash.
  • The high-voltage battery may not disconnect when the vehicle is turned off, potentially causing a slow drain.
  • In some cases, the vehicle may stall or refuse to restart after being shut down.
  • The engine may run continuously or the hybrid system may default to a limited-power mode.

Common causes, most likely first

  1. A faulty battery power-off relay or contactor assembly that is stuck open or has high resistance in its control circuit.
  2. Damaged or corroded wiring in the power-off circuit, such as a broken wire or a short to voltage.
  3. A poor ground connection at the relay, the battery management system, or the chassis point serving the circuit.
  4. A failed battery management system (BMS) or hybrid control module that is incorrectly driving the power-off line high.
  5. Water intrusion or corrosion in a connector related to the high-voltage battery pack or its control harness.
  6. A blown fuse or a faulty relay driver inside the hybrid control module, though this is less common.

How to diagnose and fix it

  1. Scan all modules for stored codes and freeze-frame data, then clear the code and test-drive to see if it returns immediately or only under specific conditions.
  2. Inspect the power-off circuit wiring and connectors for chafing, corrosion, or loose pins, especially near the high-voltage battery and under the hood.
  3. Check the ground connections for the battery management system and the power-off relay, cleaning and tightening any that show resistance.
  4. Measure the control voltage at the power-off relay with a multimeter; compare it to the manufacturer's specification to confirm whether the high signal is real or a false reading.
  5. Test the power-off relay or contactor by applying the specified control voltage and listening for a click, or by checking continuity across its power terminals when commanded.
  6. If the relay and wiring test good, substitute a known-good hybrid control module or BMS only after confirming power and ground to the module are correct.
  7. After repairs, clear the codes and perform a full hybrid system reset and road test to verify the power-off circuit operates correctly under all conditions.

Before you replace the hybrid battery pack

  • A high voltage on the power-off circuit often comes from a simple unplugged or partially seated connector at the battery pack, not from a failed internal component.
  • Technicians sometimes replace the high-voltage battery pack when the real fault is a chafed wire in the harness that rubs against the battery tray or a body seam.
  • The code can be set by a weak 12-volt auxiliary battery that causes the hybrid control module to log false high-voltage signals during cranking or low-voltage events.
  • A stuck-open power-off relay may not prevent the vehicle from driving normally, so the code can be overlooked until the high-voltage system fails to shut down, which is a safety concern.
  • Always verify the hybrid control module's software is up to date; some manufacturers released calibration updates that correct false P0A29 detection logic.

Can I keep driving with P0A29?

Do not continue driving with this code because a power-off circuit fault can prevent the high-voltage battery from disconnecting in a crash or when the vehicle is off, creating a serious safety risk. Have the vehicle inspected by a qualified hybrid technician before further use, and avoid parking it indoors or near flammable materials until the fault is resolved.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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