P0442 — Evaporative emission system — small leak detected

P0442 indicates a relatively small leak detected by the EVAP containment monitor. It does not identify the leak site: filler closure, lines or a valve that fails to seal may be involved. Thresholds and test methods are vehicle-specific; GM bulletin 21-NA-124 is not a universal diagnostic procedure.

Affected system: Evaporative emissions

Symptoms

• Engine warning lamp, usually without drivability symptoms.
• Some vehicles show a filler warning, which does not prove a defective component.

Possible causes

• Cap seal or capless filler closure failing to seal, if fitted.
• Small split or loose EVAP pipe or canister coupling.
• Purge or vent valve failing to seal when the monitor expects closure.

What to check

• 1. Record freeze frame, related DTCs and EVAP monitor status; identify the vehicle-specific system.
• 2. Check filler closure and seal per handbook; correct a loose cap before ordering parts.
• 3. Inspect accessible EVAP pipes and fittings; a small leak may have no visible crack.
• 4. Workshop: test purge and vent sealing separately with approved controls; a leaking valve is not the same as a broken hose.
• 5. Use manufacturer-approved smoke or limited vacuum/pressure tests within model limits; some leaks appear only under vacuum. No flame or improvised compressed air.
• 6. Repair only a confirmed defect, then verify the monitor and code recurrence under OEM conditions. If no leak can be established, investigate the detection strategy.

Also written as: P0442-00, P044200, PO442 (some scan tools add the fault-type byte, e.g. -00).