How do I find the leak behind a P0442 or P0455 EVAP code?
Short answer: P0455 points to a large leak and P0442 to a small one in the fuel vapor system; check the gas cap and filler neck first, then use a smoke machine to find the leak.
The EVAP system traps fuel vapors and periodically tests itself for leaks, either by pulling vacuum or by pressurizing the system, depending on the design. P0455 means a large leak, such as a missing or loose cap or a disconnected hose. P0442 means a small leak, often a pinhole or cracked seal.
Start with the cap: inspect the seal and test it with an adapter if you have one. Look at the filler neck, which rusts on many trucks, and check the purge and vent valves. Many scan tools can command the vent valve closed so you can seal the system for testing.
Then smoke test. Introduce smoke at the EVAP service port per the machine's instructions, at low pressure, and look for smoke escaping. A bright light and the machine's flow gauge help find tiny leaks. Use a machine designed for EVAP testing and never use shop air or an open flame near fuel vapor; inert-gas or EVAP-rated smoke machines are built for this.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.