What Does P0AA5 Mean?
The hybrid battery management system has detected that the negative high-voltage contactor is stuck in the open (disengaged) position and will not close when commanded. Without the negative contactor closing, the HV battery circuit is incomplete and the hybrid drive system cannot operate. The vehicle will fail to enter READY mode or will operate without hybrid assist.
Safety Warning
A stuck-open negative contactor prevents hybrid system operation but keeps the HV system safely isolated. The vehicle may be undrivable or operate in a degraded engine-only mode. Tow to a qualified hybrid service facility if the vehicle cannot maintain safe operation.
Common Causes
30%
Failed negative contactor coil (open winding preventing magnetic engagement)
25%
Open circuit in the contactor coil control wiring (broken wire or disconnected connector)
20%
Mechanical binding or contamination preventing contactor armature from moving to the closed position
15%
Battery ECU negative contactor driver failure (no output to coil)
10%
Low 12V auxiliary battery voltage insufficient to actuate contactor coil
Diagnostic Steps
1
Check the 12V auxiliary battery voltage — it must be above 12.0V under load to reliably energize contactor coils. A severely depleted auxiliary battery is a common cause of contactor engagement failures.
2
Use the scan tool to verify the ECU is commanding the negative contactor closed and check the feedback response. If the command is present but feedback shows open, the issue is downstream.
3
Measure voltage at the negative contactor coil connector pins while the ECU commands it closed. Presence of 12V with no engagement indicates a coil or mechanical failure.
4
Measure coil resistance at the connector (typically 20-100 ohms). An open-circuit reading confirms the coil has failed and the contactor assembly needs replacement.
5
If voltage is not reaching the coil, trace the wiring from the ECU driver output to the contactor connector for open circuits, damaged wires, or disconnected connectors.
Estimated Repair Cost
$500 - $3,000
Parts + labor, varies by vehicle and location
The hybrid battery management system has detected that the negative high-voltage contactor is stuck in the open (disengaged) position and will not close when commanded. Without the negative contactor closing, the HV battery circuit is incomplete and the hybrid drive system cannot operate. The vehicl...
The most common cause of P0AA5 (Hybrid Battery Negative Contactor Circuit Stuck Open) is: Failed negative contactor coil (open winding preventing magnetic engagement)
Typical repair costs for P0AA5 range from $500 to $3,000, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A stuck-open negative contactor prevents hybrid system operation but keeps the HV system safely isolated. The vehicle may be undrivable or operate in a degraded engine-only mode. Tow to a qualified hybrid service facility if the vehicle cannot maintain safe operation.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0AA5 to identify the root cause.
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Quick Info
Category
Powertrain
System
High-Voltage Battery System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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