Code Definition

P0AA4 (229) — Hybrid battery negative contactor circuit stuck closed

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Toyota

About P0AA4

P0AA4 is a generic (obd-ii standard) powertrain diagnostic trouble code recorded for Toyota vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0AA4 is stored, the module has detected an electrical-circuit fault in the wiring, connector, or driver associated with the named component.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0AA4 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

Subcode 229 is triggered when the hybrid vehicle control ECU or battery ECU detects that the negative contactor (relay) in the high-voltage battery remains closed after being commanded to open, based on inferred current flow or voltage drop. Unlike subcode 228, which may rely more directly on contactor feedback or circuit continuity, subcode 229 typically indicates continued current presence or HV system voltage retention after shutdown, implying the negative contactor did not physically open as intended.

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