01 The Hidden Safety Feature In Modern Cars That Most Owners N… • 02 The Real Reason SUVs Are Dominating the Market • 03 How to Fix an Alternator that is Draining a Car Battery • 04 If You Can't Afford Repairs, You Can't Afford the Car • 05 What Happens When Car Safety Systems Fail • 06 How to Schedule Your Automotive Maintenance • 07 Understanding Engine Thermostat: A Comprehensive Guide • 08 The Most Common Car Repair Mistake and How to Avoid It • 09 Ford F-150: The King of Trucks or a Mere Pretender? • 10 All-Wheel Drive vs. Four-Wheel Drive: Navigating Different … •

DTC codes / U35xx · Network communication / U355F

U355FCathode Backpressure Valve Control Circuit

The U355F code indicates that the powertrain control module (PCM) has detected an abnormal condition in the control circuit for the cathode backpressure valve, which is part of the fuel cell system on hydrogen-powered vehicles. The PCM monitors the circuit for expected voltage or current and sets this code when the signal falls outside the manufacturer's specified range, often due to an open, short, or component fault.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaNetwork communication

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the dashboard
  • Often no symptoms besides the warning light, especially if the valve fails to a default position
  • Reduced power or a limp-home mode if the valve is stuck and affects fuel cell performance
  • Rough running or hesitation during acceleration due to improper backpressure regulation
  • Possible fuel cell system shutdown or inability to start in severe cases

Common causes, most likely first

  1. Faulty cathode backpressure valve solenoid or actuator
  2. Damaged or corroded wiring harness in the valve control circuit
  3. Loose or disconnected electrical connector at the valve or PCM
  4. Failed PCM driver circuit for the valve control
  5. Contamination or mechanical binding of the valve itself, preventing proper movement
  6. Water intrusion or corrosion in the connector causing intermittent signals

How to diagnose and fix it

  1. Use a scan tool to retrieve all stored codes and freeze-frame data, then clear the code and test-drive to see if it returns.
  2. Visually inspect the cathode backpressure valve, its connector, and the wiring harness for damage, corrosion, or loose pins.
  3. Measure resistance and voltage on the control circuit at the valve connector and compare to manufacturer specifications.
  4. Command the valve on and off with a bidirectional scan tool to verify mechanical operation and listen for actuation.
  5. Check for technical service bulletins (TSBs) or software updates for the PCM that may address false setting of this code.
  6. If the circuit tests good, remove the valve and inspect for carbon buildup or mechanical binding; clean or replace as needed.
  7. If all components test within spec, suspect an internal PCM fault and perform pinpoint tests before replacement.

Before replacing the backpressure valve

  • A loose ground connection at the fuel cell control unit can cause this code to set intermittently, mimicking a failed valve.
  • The cathode backpressure valve often fails due to contamination from the fuel cell system, so replacing it without cleaning the passages may lead to a repeat failure.
  • This code can be set by a low battery or charging system issue that causes voltage spikes, so always verify battery health first.
  • On some vehicles, the valve is part of an assembly that includes the sensor, and replacing only the solenoid may not solve the problem.
  • A stuck-open valve may not trigger drivability symptoms immediately, leading drivers to ignore the code until performance degrades significantly.

Can I keep driving with U355F?

You can usually continue driving with this code if the vehicle runs normally, but avoid heavy acceleration or long trips until it is diagnosed. If the vehicle enters limp mode or shows reduced power, seek service promptly to prevent further fuel cell system damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage