Diagnostic article Generic OBD-II
P077C: output-speed circuit voltage drops below its electrical boundary
P077C is the output-speed sensor circuit-low diagnostic.
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Documented application
Documented monitors in gm-2025-03a, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently.
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.
P077C is the output-speed sensor circuit-low diagnostic. The cited TCM evaluates raw sensor voltage below 0.2500 volt as a ground-short signature. This is an electrical measurement rather than a test of whether output shaft RPM is low.
No-signal and circuit-low are different observations
A stationary output shaft can legitimately produce little speed activity, but that alone does not prove its signal circuit is below the electrical boundary. The raw voltage channel and the reported rotational speed must be interpreted separately. The source's detection specification associates the low-voltage condition with less than 0.5 ohm to controller ground.
The decision uses an accumulated structure: a failure interval exceeding fifty milliseconds updates a failure count, and that count must exceed sixteen. Do not reduce this to a universal fifty-millisecond DTC setting time.
Qualifying supply conditions
Run/crank and battery voltage must both exceed 10 volts for more than five seconds. Service fast learn and code-clear activity are excluded, and an active P077D prevents the shown evaluation. Those prerequisites determine whether a later normal reading actually accompanies an enabled test.
Verification should establish a valid output-sensor circuit voltage and a passing monitor under the correct conditions. A later nonzero RPM reading is useful but insufficient by itself. The table does not prove the sensor assembly is defective or authorize resistance measurement on its powered circuit.
Source and applicability
Documented monitors in gm-2025-03a, model year 2025. These are module/calibration examples; vehicle, engine and transmission applicability must be confirmed independently. GM 2025-03a, page 1118.
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
Output Shaft Speed Sensor Circuit Low
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