Code Definition

P0A78 (304) — Drive motor A inverter fail signal circuit error

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 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 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 uses a fail signal line to notify the MG ECU of internal protection events such as overvoltage, overcurrent, or thermal faults, and the two exchange this signal on a defined timing. DTC P0A78 with this sub code sets when the MG ECU fails to receive the fail signal within its expected time window during inverter startup, operation, or self diagnosis. This indicates a timing or communication integrity problem, typically an intermittent or delayed signal from degraded wiring, a poor connection, electrical noise, or an inverter responding late, rath...

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