P0455EVAP system large leak detected

Means the evaporative emissions (EVAP) monitor could not build or hold the expected vacuum/pressure—the leak is in the “large” bucket, typically on the order of a loose or missing gas cap, an open purge/vent path, or a disconnected EVAP hose. Unlike a small-leak P0442, this one often sets after one incomplete seal at the filler neck. Drivers usually see only a check-engine light and a failed EVAP readiness flag; fuel smell at the filler or a missing cap is the giveaway when present. Freeze frame rarely shows a drivability complaint because EVAP is a sealed vapour system, not a fuel-trim problem.

Start where fleets and shops waste the least time: the gas cap. Confirm the correct locking cap is present, the seal is pliable, and the filler neck sealing surface is clean—then tighten to the click and retest. If the cap is honest, move to purge and vent hardware. A purge valve (canister purge solenoid) stuck open can yank a huge vacuum hole into the intake and still fail the large-leak monitor; a vent valve (canister vent solenoid) stuck open or a chewed vent hose dumps the tank to atmosphere. Loose quick-connects at the charcoal canister, cracked plastic lines along the frame, and a damaged fuel-tank pressure (FTP) sensor reading that never moves are the next common finds. Pinholes that only smoke under low pressure belong more to P0442—do not confuse the two monitors.

Diagnosis is seal-first, smoke-second. Capture freeze frame and note whether purge/vent commanded states were open during the set. Command the purge valve closed and the vent closed (or follow the OEM sealed-system test), then watch FTP sensor voltage/kPa while the ECM tries to pull vacuum. No vacuum build with both valves commanded closed means a large open path—cap, hose, or vent stuck open. Vacuum that builds then collapses points to a big leak you can often hear or smell. Smoke the EVAP system at the OEM-specified low pressure only; high shop air will blow seals you did not have. Trace smoke at the cap, canister, purge line into the intake, and vent filter/hose. Verify FTP sensor sweeps when you apply a known pressure/vacuum with a hand pump before condemning valves.

Replace the gas cap or reconnect the obvious hose when that restores FTP behaviour and the monitor passes. Dig deeper when a new cap still fails, when FTP stays flat with valves proven closed, or when smoke only appears at a micro-orifice (that is P0442 territory). System context: EVAP stores and meters tank vapours through the charcoal canister into the intake under purge—this is not a lean-bank fuel code and not a thermostat issue. Cross-check related small-leak (P0442) and general EVAP (P0440) history; a large leak fixed with a cap often clears the family after a complete drive cycle.