2002 LS1 EVAP Leak / Purge / Vent Diagnosis
2002 LS1 EVAP diagnosis that separates large leaks, small leaks, purge, vent-path restriction and fuel-tank-pressure feedback before parts replacement.
The PCM creates and watches vacuum.
For the large-leak test the PCM closes the vent, opens purge and watches the fuel-tank-pressure sensor to see whether the system can reach vacuum. It then closes purge and watches how quickly vacuum decays. The small-leak test follows only after the large-leak test passes.
An EVAP code does not automatically mean gas cap. The cap is one possible leak point in a system that also includes hoses, canister, purge, vent, tank/filler components, wiring and pressure feedback.
- P0440 / large leak: verify the system can pull and hold vacuum; inspect gross leaks/blockages and commanded purge/vent operation.
- P0442 / small leak: diagnose sealing/decay after the large-leak test passes.
- Vent restriction behavior: a blocked/restricted vent path can pull excessive tank vacuum rather than simply leaking.
- Modified fuel system: identify aftermarket tank/module, relocated/deleted EVAP hardware and calibration changes before using the stock tree.
Factory service information uses regulated inert-gas EVAP test equipment and leak detection. Do not improvise by pressurizing a fuel-vapor system with shop air or an unsafe pressure source.
AFTER THE FAILURE IS PROVEN
Use the proven failed branch—not the original symptom—to choose the replacement. Go from diagnosis to the Parts Verdict →