P2B87: Coolant pump A reports speed below its requested operation

P2B87 is the coolant-pump underspeed diagnosis in the reviewed GM strategy. The narrative defines it as reported pump speed slower than commanded speed, using a filtered comparison between command and feedback.

The numerical sign needs source caution

The table describes the error as command minus feedback, but the published malfunction comparison uses a less-than symbol for underspeed. That conflicts with the straightforward positive-error interpretation of a slower actual speed. This article therefore does not turn the displayed comparator into a verified service threshold.

The source also divides criteria at 12 V because feedback speed depends on supply voltage. The applicable calibration and service information are needed to resolve exact limits; a guessed universal rpm shortfall would be misleading.

Several useful conditions are clear

Command changes must remain below 50 rpm between samples for at least three seconds. The pump must be enabled with command in the 300–3,480-rpm range. Supply qualification, valid speed feedback and temperature-related delays also apply. Failure must occupy eight seconds within ten seconds.

These conditions distinguish a sustained tracking error from normal response immediately after a changing command. Record command, actual speed, voltage and feedback status on the same timeline.

Verification should demonstrate proper speed tracking through the applicable completed monitor after the numerical criterion is confirmed. P2B87 does not itself measure coolant flow or prove an obstruction, air pocket or defective pump. Type B, two-trip behavior is documented.

Source and applicability

This description is limited to the stated monitor; vehicle, engine and calibration matching remain necessary. GM diagnostic table, PDF page 677. GM diagnostic table, PDF page 678. GM diagnostic table, PDF page 679.