Code Definition

P0AA4 (228) — 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

This DTC is set when the battery smart unit or hybrid vehicle control ECU detects that the negative contactor (relay) in the high-voltage (HV) battery pack is remaining closed when it should be open. Subcode 228 specifically refers to a failure condition where the ECU commands the negative contactor to open (typically during shutdown or isolation events), but voltage or current sensing indicates that the circuit remains closed, suggesting a mechanical or electrical failure of the contactor or its control circuit.

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