U355F
Cathode Backpressure Valve Control Circuit.
- Generic
- Network
- Generic code
- Medium severity
How this code is built
- U
- SystemNetwork
- 3
- Defined bySAE, the same on every car
- 5
- AreaNetwork area 5
- 5F
- FaultThe specific fault in that area
What it means
U355F with the generic definition 'Cathode Backpressure Valve Control Circuit' points to a fault in the control circuit of the valve that regulates air pressure on the cathode side of a fuel cell stack. The control unit has seen the valve's command, feedback or communication signal outside the expected range, or lost the valve's response. This only applies to fuel cell vehicles, and the exact wording and conditions for setting the code vary by model and system design.
Symptoms
- Fuel cell or powertrain warning lamp on
- Reduced power or limp-home mode
- Vehicle refuses to start or shuts the fuel cell system down
- Reduced range or poor efficiency
- Fault message on the dashboard
- Other stored codes for the air supply or fuel cell stack
Possible causes
- Damaged wiring, poor connector contact or corrosion in the valve control circuitlikely
- Faulty cathode backpressure valve actuator or internal position sensorlikely
- Valve seized or sticking due to contamination, moisture or icepossible
- Low supply voltage or poor earth to the valve or its driverpossible
- Fault in the fuel cell control unit driver stage or a communication fault with the module controlling the valvepossible
- Outdated or faulty control unit software where the manufacturer has a known fixrare
How to diagnose
- Read all stored and pending codes and note the freeze frame data, including ambient temperature and operating state when the code set.
- Check for related codes covering the air supply, stack pressure sensors or network communication, and deal with those first if they appear.
- Look up the manufacturer's procedure for the model, including the high-voltage and hydrogen safety precautions. Only suitably trained staff should work on these systems.
- Inspect the valve wiring, connectors and earth points for chafing, water ingress, corrosion and loose pins.
- Check the supply voltage and earth at the valve connector against the manufacturer's data.
- Using a suitable scan tool, command the valve and compare commanded and actual position or feedback.
- Check the valve for physical sticking, contamination or freezing, following the manufacturer's procedure.
- Check whether a software update applies to the vehicle, and only apply it if the manufacturer lists it for this fault.
How to fix
- Repair or replace damaged wiring, terminals and connectors, then clear the code and retest.
- Replace the cathode backpressure valve if it fails the actuation or feedback test.
- Clean or free a sticking valve only if the manufacturer permits it, otherwise replace it.
- Restore correct supply voltage and earth connections.
- Replace or repair the control unit only after the wiring and valve have been proven good, following the manufacturer's procedure.
- Apply a manufacturer software update where one is a known fix for this fault.
Can I keep driving?
Avoid long journeys and get it checked soon, as the vehicle may be in reduced-power mode and the fault could affect the fuel cell stack. If the vehicle warns you to stop, shuts down or smells of anything unusual, stop safely and seek qualified help.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.