P0455 — EVAP system large leak detected

Part of system: Aftertreatment & emissions systems

P0455 often shows up through monitors and inspection readiness more than a sudden roadside failure. P0455 indicates a large evaporative-emissions leak—the ECM could not build or hold vacuum/pressure in the EVAP system during the monitor, implying an opening bigger than a small-leak threshold. The classic cause is a missing or untightened gas cap, but disconnected purge lines, cracked canisters, or open vent valves also qualify. Start with the cap, then run a smoke test at low pressure and watch the FTP sensor while commanding the vent closed. Typical contributors include missing, loose, or non-sealing fuel filler cap, disconnected or split EVAP hose at the engine or canister, and vent valve stuck open to atmosphere. A calm sequence is to tighten or replace the cap and rerun the OEM EVAP test, then smoke-test the system and repair the first large escape point, and only afterward budget for oEM fuel filler cap if the data still points there.