P2C29: Detecting a stuck bypass valve from pressure pulsations

P2C29 identifies a suspected stuck-closed turbocharger bypass valve in the documented strategy. The controller looks for pressure pulsations while the valve is commanded on. It is an indirect response test, rather than a statement that a dedicated position sensor measured the valve closed.

Counted pulses with qualified inputs

The published criterion is more than ten pulsations over at least one second. A pulse is associated with downstream-compressor pressure after high-pass filtering exceeding 10 kPa for 0.1 second. The table also describes a flow-ratio threshold sequence, although part of that explanatory sentence is incomplete.

The valve command and output stage must be active. Intake-air temperature must exceed 14.96°C, and both air-mass and intake-temperature information must be valid. The diagnostic scheduler must enable the test. These conditions prevent a pressure fluctuation captured outside the commanded test from being treated as identical evidence.

Interpret response before selecting a part

Preserve the valve command, pulse count and validity of the supporting inputs. A repeated pressure pattern under those conditions supports the controller's stuck-valve suspicion; it does not independently distinguish every mechanical, control or measurement cause.

The incomplete flow-ratio description is a reason to obtain the matching diagnostic procedure before attempting to recreate the waveform test. Do not improvise a boost event to force the code. Verification should show the scheduled response check completing with acceptable behaviour, rather than relying on one apparently normal pressure reading or replacing a bypass valve solely from the code title.

Source and applicability

2025 GM 07A-2 documented monitor only; module and calibration applicability must be matched to the vehicle. The application-matrix filename join remains unresolved; no model or engine fitment is inferred. GM PDF, page 858.