P0626: The high F-terminal test belongs to a stationary key-on starting state

P0626 identifies a high generator field/F-terminal circuit condition. In the cited GM calibration, this is a key-on test of the filtered F-terminal duty cycle. Its operating window is materially different from a general charging-system assessment while the engine is running.

The expected state is part of the threshold

The published failure criterion is filtered duty cycle above sixty-five percent for five seconds. The enable conditions specify ignition on, the controller in starting mode and engine speed equal to zero. Vehicle speed must remain below 0.62 mph, and crank and cam sensor fault indications must be absent.

Those conditions explain why a high duty reading during established engine operation does not demonstrate the same P0626 event. The signal is being evaluated against the expectations of the defined key-on state, not against one universal duty limit for all engine loads.

Preserve starting-state evidence

Record the filtered F-terminal value with engine speed, controller mode and the relevant sensor-fault status. A trace captured only after startup may omit the exact condition the monitor used. Similarly, invalid speed or phase information can undermine whether the documented operating state has been established.

The row lists one-trip reporting, but does not identify an alternator, wiring connection or controller as the failed part. It also supplies no direct terminal-probing procedure. The matching vehicle charging and signal-circuit information is required before turning the high-duty finding into a repair decision.

Source and applicability

2025 GM 07A-1 documented monitor only. Vehicle, engine and controller calibration applicability must be established separately; no model fitment is inferred from the file. GM PDF, physical page 924.