Network, generic (extended)

U355F Cathode Backpressure Valve Control Circuit

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Open, short or corrosion in wiring or connector of the valve actuator (wet exhaust area)
Valve motor or actuator winding faulty
Driver stage of the fuel cell control unit faulty or software outdated

Symptoms

  • Fuel cell system warning, MIL on
  • Noticeably reduced power
  • Fuel cell system shuts down or vehicle does not reach READY

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€150–1,200
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

cathode backpressure valvevalve wiring

Related codes

U3560U3561U3562U355D

References: patents.google.com, patents.google.com, static.nhtsa.gov.