P28F7: excess exhaust pressure and the exhaust throttle position

The GM monitor for P28F7 checks whether exhaust pressure is too high because the exhaust throttle valve is in the wrong position. The source gives a broken linkage between valve shafts, leaving the valve closed, as one example. That example is a possible mechanism, not a finding established by the code alone.

Pressure is the deciding evidence

The documented failure threshold is measured LPE differential pressure above 80 kPa. The decision requires 480 failing counts within 600 samples, with a 12.5-millisecond task interval. This means the monitor evaluates sustained sampled behavior rather than treating every isolated pressure peak as a confirmed failure.

It runs with the engine out of cranking, powertrain relay voltage above 11 volts, the diagnostic enabled and torque request above a calibrated minimum. Crucially, it requires no fault on the LPE differential-pressure sensor. Pressure-sensor validity therefore affects whether the exhaust-valve inference is available.

Distinguish feedback from actual movement

A useful diagnostic comparison is measured pressure against requested exhaust-throttle operation and the operating load at the event. The cited linkage example explains why apparently commanded movement need not guarantee that the pressure-controlling part of the valve moved correctly. Conversely, a questionable pressure signal should be resolved before attributing the event to the throttle mechanism.

The table does not provide a universal valve position, component location or replacement instruction. Verification should show a credible pressure response under the enabled load conditions and a completed monitor. Work around a hot exhaust system and actuator mechanisms requires the applicable service procedure.

Source and applicability

Documented 2025 GM gm-2025-04b monitor only; vehicle, engine and controller applicability must be established separately. GM monitor table, PDF page 1515.