P0455Evaporative Emission Control System Leak Detected (gross leak)

What it means

The same self-test as P0442, with the opposite result: the fuel-vapor system couldn't hold vacuum at all, meaning a large opening somewhere. The classic cause is also the simplest one — a gas cap left loose or sitting on the roof after a fill-up. The engine runs normally regardless.

Is it safe to drive?

Drive normally — check soon

Harmless to drive with. You'll lose a little fuel to evaporation, might catch a fuel smell, and will fail an emissions test until it's fixed — that's the extent of it. Fix the cap and the code typically clears itself within a few drive cycles.

Common causes

Most likely first.

• A gas cap that's loose, missing, or has a torn seal — the classic
• A disconnected or split EVAP hose
• A vent or purge valve stuck open
• A cracked filler neck — rust around the cap seat on older cars

What the live data shows

The purge duty and tank pressure PIDs behave as if the system were open to the atmosphere — purge commanded, no pressure response. After reseating the cap, watch the EVAP readiness monitor flip back to passed over the next few drives; that's the confirmation the leak is gone, no tools required beyond the scanner.

A scanner that reads all 139 standard sensors shows you these readings live. ScanMyCar charts them as you drive, keeps the freeze-frame stored with the code, and pulls codes from every module — free, with any cheap Bluetooth adapter.

Typical fixes

Cheapest first, where honesty allows.

• Reseat or replace the gas cap — resolves most cases for a few dollars
• Look for an obviously disconnected vapor hose near the tank and charcoal canister
• Replace a stuck-open vent valve
• Inspect the filler neck for rust or cracks