Code Definition

P0478 — Exhaust Pressure Control Valve High

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

About P0478

P0478 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0478 is stored, the module has detected a signal or supply voltage higher than the expected operating range.

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 P0478 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

The Powertrain Control Module (PCM) sets this code when it detects that the exhaust pressure control valve is operating at a higher-than-expected level or is stuck in a high-pressure position. This may result from an electrical issue, mechanical obstruction, or excessive backpressure in the exhaust system.

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