P0478: exhaust throttle command circuit short to power

The documented P0478 check identifies a short-to-power category in the exhaust throttle command circuit. Its hardware feedback criterion is H-bridge switch current above 8 A with the driver on.

Why the electrical category matters

P0477's short-to-ground check uses the same current threshold. Therefore, excessive current without the associated hardware status cannot tell which direction of short the controller has classified. Keep the code, command state and circuit evidence together.

This monitor is separate from exhaust-pressure measurement and from whether the valve follows a requested position. Neither high exhaust pressure nor a large valve-angle mismatch alone reproduces its electrical criterion.

Eligible monitoring and confirmation

Relay supply must exceed 11 V, cranking must be over and diagnostics must be enabled. Hardware status cannot be indeterminate. The source specifies 240 failed counts among 300 samples on a 12.5 ms task, with Type A reporting after one trip.

A retest should establish normal behavior during an eligible driven period, followed by whatever circuit verification the vehicle's service procedure requires. A test only at the fully open default with the motor unpowered does not exercise this current check. The table cannot select a harness, actuator or controller replacement and does not authorize applying power or bypassing protection to imitate a fault.

Source and applicability

2025 GM 25OBDG04B diagnostic parameter tables; the cited monitor only. Vehicle, engine and calibration applicability must be confirmed separately. These tables describe detection logic, not a complete repair procedure or a component replacement verdict. gm-2025-04b, PDF page 1199.