The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. Plug-in hybrids and range extenders with a sealed, pressurised tank have an electric valve in the vapour line between tank and canister. It is normally closed and opens before refuelling, so that the tank is depressurised – the filler flap is only released then – and refuelling vapour reaches the canister (Ford patent). P04B0 means the valve's control circuit is open. The valve then does not open, the tank cannot be depressurised and the filler flap may not open or only after a delay. No manufacturer document available to us uses this code – Ford reports its tank isolation valve as P2418/P2450 – so the description follows CARB and the component patent.
Low severity. Can I keep driving?You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.
Symptoms
- Check engine light on
- Filler flap does not open after pressing the refuel button, or only after a delay
- EVAP readiness not set
Most likely causes
- Connector at the tank valve (tank, underbody) loose or corroded
- Valve coil open
- Supply or control wire open
- Driver in the control module
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P04B0 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 4: Emissions
- B0: Specific fault in that system
Frequently asked questions
Can I keep driving with code P04B0?
You can normally keep driving. If it affects the fuel level, do not trust the gauge and refuel earlier than usual.
What causes code P04B0?
The most common causes are: connector at the tank valve (tank, underbody) loose or corroded; valve coil open; supply or control wire open.
Related codes
- P04B1Refueling Vapor Control Valve Circuit Low
- P04B2Refueling Vapor Control Valve Circuit High
- P04B3Refueling Vapor Control Valve Performance/Stuck Open
- P2418EVAP System Switching Valve Control Circuit/Open
- P2450EVAP System Switching Valve Performance/Stuck Open
- P0449EVAP System Vent Valve Control Circuit/Open
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.