U355F code meaning
Cathode Backpressure Valve Control Circuit
Network system Generic code U35xx codes Check engine light
What U355F means
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.
U355F at a glance
- Check engine lightYes, usually on
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyWorkshop only
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U355F
- Fuel cell system warning, MIL on
- Noticeably reduced power
- Fuel cell system shuts down or vehicle does not reach READY
Common causes of U355F
- Most likelyOpen, short or corrosion in wiring or connector of the valve actuator (wet exhaust area)
- PossibleValve motor or actuator winding faulty
- Less likelyDriver stage of the fuel cell control unit faulty or software outdated
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Cathode backpressure valveValve wiring
What each part of U355F means
| U | Network system: communication between the control modules (CAN bus). |
| 3 | Reserved range, usually generic. |
| 5F | The exact fault in that area. |
What to do next
- Note U355F and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U355F
What does U355F mean?
U355F means "Cathode Backpressure Valve Control Circuit". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U355F?
Drive gently and only to a workshop. With U355F the car can go into limp mode (reduced power) to protect itself.
Will U355F fail an emissions test?
U355F is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U355F to fix?
Difficulty: workshop only. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will U355F clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.