P0496: Tank vacuum develops even though purge is commanded closed

P0496 checks for EVAP flow during a non-purge condition. In the cited GM monitor, both purge and vent valves are commanded closed, and the controller watches for tank vacuum increasing because the purge path leaks toward intake-manifold vacuum.

The leak direction is important

The documented failure is tank vacuum above 2,491 Pa for five seconds during the permitted test period. Test time accumulates only when engine vacuum exceeds 10 kPa. A weak manifold-vacuum condition may therefore prevent a meaningful elapsed test even while ordinary clock time passes.

The source explicitly distinguishes leakage toward the intake from leakage to atmosphere. P0496 targets the former; it should not be interpreted as a generic proof of every possible EVAP sealing defect.

A cold-start opportunity is required

This calibration specifies more than 28,800 seconds of engine-off time, restricted startup temperatures and fuel level, and a once-per-cold-start schedule. Pressure, purge/vent circuit and communication faults can inhibit the test.

Capture closed-valve commands, tank-vacuum trend and manifold-vacuum eligibility together. Unexpected vacuum supports investigating the purge path, but the table does not replace the vehicle's valve and circuit isolation procedure. Verification should show a completed non-purge flow test without the qualifying vacuum rise. An ordinary hot restart or a passing electrical valve check does not reproduce this sealing-performance evaluation.

Source and applicability

This explanation is limited to the 2025 GM 25OBDG03A monitor on page 308, page 309. Its thresholds are calibration examples, not universal specifications. Confirm the vehicle, engine and controller procedure before applying them.