Code Definition

P0A78 (523) — Drive motor A inverter performance

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

About P0A78

P0A78 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 P0A78 is stored, the module has detected a plausibility fault — the measured value is in the electrical range but is not consistent with expected operating behaviour.

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 P0A78 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

On this Toyota hybrid the Drive Motor A inverter is liquid cooled and reports its temperature to the MG ECU so the system can protect the power electronics from overheating. DTC P0A78 with this sub code sets when the inverter internal temperature control or thermal monitoring fails, either by exceeding critical temperature thresholds without initiating protection or by providing invalid thermal feedback to the ECU. Common causes include a faulty inverter temperature sensor, a thermal protection circuit fault, a cooling system problem such as low coolant, an air lock, a flow restriction, or a f...

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