P2B85: Coolant pump underspeed is a sustained tracking deficit

P2B85 evaluates a coolant pump that reports less speed than the controller requested. The GM algorithm filters command minus feedback and compares the positive deficit with a calibrated threshold. A low absolute speed is not enough when the command itself is also low.

Establish stable demand before measuring the deficit

The commanded speed must change by less than 50 rpm for at least three seconds. The continuation page also requires pump enablement, command at least 300 rpm and supply eligibility after initialization. These conditions separate a sustained failure to follow demand from an ordinary response during a rapidly changing request.

The confirmation window is eight seconds of failure within ten seconds. A brief lag and a completed underspeed result therefore represent different evidence. Keep command, actual speed and the filtered relationship together over time.

Recognize the calibration gap

The published table names normal- and low-voltage thresholds without providing usable numerical values for them. It also prints a −9,999 V branch boundary. That entry cannot be treated as a physical vehicle supply limit or silently corrected into an assumed voltage.

The supported conclusion is persistent underspeed relative to stable demand. The summary does not distinguish a pump mechanism problem from feedback or control issues, and it does not prescribe replacement. The actual calibration and vehicle procedure are needed before reproducing the precise numerical pass/fail decision.

Source and applicability

2025 GM 25OBDG03B documented monitor only. Vehicle, engine and calibration applicability must be confirmed from the matching service information; no model fitment is inferred. These criteria explain controller logic, not a complete repair procedure. 25OBDG03B, PDF page 880. 25OBDG03B, PDF page 881.