P2CBB: fan 3 underspeed is a model-based performance result

P2CBB evaluates cooling fan 3 for underperformance. Its expected speed is built from the fan request with startup, transport and ramp delays, rate limiting and voltage compensation. Actual speed must remain within a calibrated low-side margin of that expectation.

Fan 3 has its own qualifications

The table requires fan speed at least 875 rpm, fan command on and battery voltage within range. It also requires LIN-based operation enabled and excludes fan-3 communication code U067C, continuous-operation code P135E and speed range codes P30F3/P30F2. These exclusions prevent a missing or invalid speed signal from being treated as an ordinary performance shortfall.

Confirmation uses 16 failed observations out of 20 one-second samples. The continuation explains that the controller can hold the fan commanded on after initial diagnostic activation to give the monitor time to reach a decision.

Useful comparisons

Review measured speed against the modeled or applicable calibrated expectation after permitted response delays. A brief lag immediately after a command increase is not the same evidence as a persistent deficit once the expected response has settled. Do not borrow fan 2's minimum-speed qualification merely because its monitor has a similar name.

Verification needs a valid fan-3 feedback signal and completion of this comparison while its prerequisites are met. The source does not specify a universal fan speed for every command percentage, or prove whether an underspeed event originates in motor drive, electrical supply or another part of the fan system.

Source and applicability

Documented 2025 GM gm-2025-04b monitor only; vehicle, engine and controller applicability must be established separately. GM monitor table, PDF page 882; GM monitor table, PDF page 883.