Diagnostic article Generic OBD-II
P27C8: Valve-three circuit high can be retained from a shutdown test
P27C8 combines active driver diagnostics with a shutdown check for coupling between low-side output circuits.
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Documented application
2025 GM 25OBDG07A Part 2 documented monitor only; its application-matrix filename join is unresolved. No model, engine or universal calibration applicability is inferred.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
For P27C8, the source identifies synchronizer actuation valve three. During operation, its electrical diagnostic checks a supply-dependent output voltage and disagreement between requested current and an estimated current derived from driver behavior. A fixed voltage alone is therefore an incomplete account of this DTC.
A second route occurs at shutdown
The controller also tests for an unwanted connection between low-side outputs by activating them one at a time. A reciprocal fault pattern when the two commands exchange states indicates coupling. That result is written to nonvolatile memory and reported at the following startup.
A code noticed immediately after starting may therefore describe a previous shutdown event rather than a continuously high valve-three signal at that moment.
Preserve which route failed
Keep the active-output diagnostic status or retained shutdown-pair information with the code. A normal energized-current trace does not necessarily clear the cross-circuit route, while a clean shutdown result does not by itself explain an earlier active current-estimation failure. The source’s generic solenoid-P name requires controller-specific mapping. Applicable service information is needed to identify the circuits and isolate the defect; the code does not reveal a specific neighboring wire or failed part by itself.
Document scope and source
2025 GM 25OBDG07A Part 2 documented monitor only; its application-matrix filename join is unresolved. No model, engine or universal calibration applicability is inferred. These figures explain the cited controller logic, not a vehicle-specific repair instruction. Match the vehicle’s service information before applying them.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Pressure Control Solenoid P Control Circuit High
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT