P0299: Underboost at steady demand or during a pressure rise

P0299 indicates insufficient boost in the cited GM pressure-control strategy. It considers two situations: actual pressure falling short of steady demand, and pressure rising too slowly during a transient. The section excludes the mechanically driven supercharger case identified in its note.

Two evidence patterns

For steady operation, the narrative uses desired pressure minus actual pressure and describes a positive difference as underboost. The allowed deviation depends on engine speed, desired boost and ambient pressure. The steady path counts 40 failures in 50 samples at 100 milliseconds each.

The transient path evaluates boost-pressure change and uses a separate 14-of-20 sampling decision. Consequently, reaching the desired pressure eventually does not prove the pressure-rise evaluation passed. Record both the demand trend and the actual response.

A comparison sign remains unresolved

The steady-state threshold cell prints a less-than sign beside a narrative describing positive underboost deviation. This article does not silently reverse the sign or publish an invented fixed limit. The applicable calibration and service procedure must resolve the comparison before numeric diagnosis.

Both paths require valid related sensor and actuator information and exclude active wastegate/recirculation device control. Check those prerequisites before interpreting a missing test as a pass. Verification should exercise the failure path that actually occurred. This code alone does not identify a turbocharger, pressure leak, actuator or sensor as the repair.

Source and applicability

This explanation is limited to the 2025 GM 25OBDG03A monitor on page 232, page 233, page 234. Its thresholds are calibration examples, not universal specifications. Confirm the vehicle, engine and controller procedure before applying them.