P24B9: EVAP pressure correlation compares the vented system with ambient pressure

P24B9 describes a correlation check between leak-detection-pump pressure and barometric pressure in the reviewed GM EVAP sequence. A crucial limitation is that the displayed calibration marks this diagnostic disabled. Its documented algorithm explains the code, but does not prove the test is active on a particular vehicle.

Two checks serve different purposes

With the vacuum pump off and switching valve vented, the controller compares the ELCP absolute-pressure reading with barometric pressure derived from the MAP sensor. A difference above 3,000 Pa increments the failure counter during a 14-second evaluation.

The first ambient check can report failure, but does not immediately report a pass. A similar final check later in the sequence must finish before the passing result is reported. An acceptable early reading therefore does not establish successful completion of the entire correlation diagnosis.

Off-state eligibility is extensive

The documented sequence uses scheduled wake-up windows hours after propulsion stops, with fuel-level, temperature, pressure and voltage restrictions. Key-up, service-bay testing, refueling and certain device-control activity can abort it. The counter criteria are 50 failures in 63 samples at 100-millisecond intervals.

Related pressure, valve, pump and communication faults also affect availability. Before interpreting a missing result, establish whether this monitor is actually enabled in the vehicle's calibration and whether the sequence reached its final stage.

Verification requires consistent ambient-pressure correlation in the applicable completed test. This table does not identify which pressure source is wrong or prove that the EVAP system has a leak.

Source and applicability

Confirm the reporting module, system and calibration before applying these diagnostic limits. GM diagnostic table, PDF page 654. GM diagnostic table, PDF page 655.