P2636: Transfer-pump performance is inferred from two fuel levels

P2636 is commonly labeled fuel pump B low flow or performance. In the cited GM LLT-specific section, it is a fuel-transfer-pump failure monitor based on two fuel-level signals. The controller does not use fuel-rail pressure or measured pump current as the stated failure criterion.

The two levels must be considered together

The listed condition combines fuel level 1 below 10.6% with fuel level 2 above 22.8%. Neither threshold alone expresses the monitored imbalance. A report showing only the dashboard fuel gauge cannot reconstruct these two individual sender observations.

The signals must be free of sensor failure and the fuel-level state must be stable. This qualification matters because a questionable sender or a changing level state undermines the comparison the monitor is intended to make. The summary does not establish the physical location of each sender or identify which tank section a generic scan-tool label represents.

The imbalance must persist

Engine speed must exceed 240 rpm and the listed evaluation interval is 240 seconds. A short snapshot of unequal levels is therefore not equivalent to the sustained condition. This table also lists no MIL for the entry, so the absence of an engine warning lamp does not prove that transfer performance was assessed successfully.

Preserve both level readings, their validity and stability status, and event duration. Verification concerns the qualified two-level relationship, not merely hearing a pump operate. The code alone cannot establish pump replacement; transfer-system diagnosis and any fuel handling require vehicle-specific professional procedures.

Source and applicability

GM 2009 09OBDG02 Engine Diagnostics, LLT-specific section, fuel-transfer-pump monitor on physical page 40. This source does not establish sender location, tank design or per-model applicability.

General Motors source, physical page 40