U355FCathode Backpressure Valve Control Circuit
- Generic OBD-II code · Network
- Turns on the check-engine light
- Can trigger limp mode
- How the car detects it
- U355F reports a wiring or output stage fault on the actuator of the valve at the cathode outlet, which throttles the air leaving the stack and thereby sets the pressure the air side runs at. GM moves such a valve in closed loop on a pressure reading, in steps or continuously. U355F itself is absent from public maker documents. For the 2021–22 Mirai, Toyota bulletin T-SB-0043-23 ties a lit MIL to a stuck air pressure regulating valve (P1F6E71), fixed by new EV control software or, since the 2025 revision, a new valve. In a Toyota patent air pressure that repeatedly exceeds about 270 kPa counts as a fault: compressor stopped, air routed into the bypass, valve opened. An open or shorted drive leaves cathode pressure uncontrolled; expect a warning and cut power. Position feedback has its own code, U3560.
- What you may notice
- Fuel cell system warning, MIL on
- Noticeably reduced power
- Fuel cell system shuts down or vehicle does not reach READY
- Likely causes
- LikelyOpen, short or corrosion in wiring or connector of the valve actuator (wet exhaust area)
- PossibleValve motor or actuator winding faulty
- RareDriver stage of the fuel cell control unit faulty or software outdated
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €150 – €1,200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.