U3562: Cathode Bypass Valve

What it means

Generic fault on the cathode bypass valve — combines circuit and mechanical performance. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

The cathode bypass valve routes air around the fuel-cell stack during startup or low-load operation, and its operation depends on both the actuator and valve mechanism. The control module sets this code when it detects a fault affecting the valve's electrical control or overall operation.

Common causes

Most likely first.

1. Valve electrical or mechanical failure
2. Gateway module routing CAN messages incorrectly
3. CAN terminating resistor (120 Ω) drifted out of spec

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Record freeze-frame data and scan all modules for valve, fuel-cell, and communication faults to establish whether the issue is isolated.
2. Inspect the bypass valve body, actuator, connector, and harness for physical damage, obstruction, corrosion, or loose connections.
3. Check the actuator's power, ground, and control circuits against the manufacturer's specification.
4. Compare commanded and reported valve position or response in live data, if the vehicle provides it, to distinguish an electrical fault from a sticking valve.
5. If communication faults span multiple modules, test gateway routing and CAN network integrity to the manufacturer's specification.

Common fixes

• Repair damaged or corroded wiring and connectors at the cathode bypass valve.
• Remove an identified external obstruction or correct a sticking valve mechanism where serviceable.
• Replace the bypass valve actuator or valve assembly if testing confirms electrical or mechanical failure.
• Repair confirmed gateway or CAN network faults.

Can I keep driving?

The vehicle may limit fuel-cell output or enter limp mode if the bypass valve cannot operate correctly. Avoid hard acceleration and stop safely if a shutdown warning or major power loss occurs.