Faulty anode header drain valve heater relay, such as a stuck open or closed coil.; Damaged or corroded wiring in the relay control circuit, including opens or shorts.; Poor electrical connection at the relay connector or powertrain control module (PCM) connector.; Failed PCM driver circuit for the relay control.; Blown fuse or fusible link supplying power to the relay circuit.; Water intrusion or contamination in the relay or connector causing intermittent faults.

DTC codes / U35xx · Network communication / U356F

U356FAnode Header Drain Valve Heater Relay Control Circuit

Q: What does code U356F mean?

Anode Header Drain Valve Heater Relay Control Circuit. The U356F code indicates that the powertrain control module has detected a fault in the control circuit for the anode header drain valve heater relay, which is part of the fuel cell system's hydrogen recirculation and water management. The computer monitors the relay control circuit for expected voltage or current and sets this code when the signal is out of range, indicating an open, short, or relay malfunction.

The U356F code indicates that the powertrain control module has detected a fault in the control circuit for the anode header drain valve heater relay, which is part of the fuel cell system's hydrogen recirculation and water management. The computer monitors the relay control circuit for expected voltage or current and sets this code when the signal is out of range, indicating an open, short, or relay malfunction.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaNetwork communication

Symptoms you may notice

Illumination of the malfunction indicator lamp (MIL) on the dashboard.
Often no symptoms besides the warning light, as the vehicle may continue to operate normally.
Reduced fuel cell system performance or power output in some cases.
Possible rough running or hesitation if the heater relay affects related components.
In rare instances, the vehicle may enter a limp mode or fail to start.
No noticeable change in driveability for many drivers.

Common causes, most likely first

Faulty anode header drain valve heater relay, such as a stuck open or closed coil.
Damaged or corroded wiring in the relay control circuit, including opens or shorts.
Poor electrical connection at the relay connector or powertrain control module (PCM) connector.
Failed PCM driver circuit for the relay control.
Blown fuse or fusible link supplying power to the relay circuit.
Water intrusion or contamination in the relay or connector causing intermittent faults.

How to diagnose and fix it

Verify the code with a scan tool and check for any related diagnostic trouble codes (DTCs) in the fuel cell system.
Inspect the relay and its connector for corrosion, damage, or loose pins, and clean or repair as needed.
Test the relay by swapping it with a known-good relay of the same part number and clearing the code to see if it returns.
Check the fuse and wiring for the relay control circuit using a multimeter to verify continuity and voltage.
Perform a voltage drop test on the control circuit to identify excessive resistance.
If the relay and wiring are good, test the PCM driver circuit with a bidirectional scan tool or oscilloscope.
Replace the faulty relay, repair wiring, or reprogram/replace the PCM as diagnosed.

Relay Circuit Checks Before PCM Replacement

A common oversight is replacing the PCM when the actual fault is a chafed wire in the harness near the relay, so always perform a thorough visual and continuity check first.
The anode header drain valve heater relay may share its control circuit with other relays or solenoids, so a fault in a related component can set this code.
Intermittent faults often occur due to vibration or temperature changes, so wiggle-test the harness while monitoring the circuit.
Some vehicles may set this code without any driveability issue, leading owners to ignore it, but it can affect fuel cell efficiency and should be addressed.
Always check for technical service bulletins (TSBs) for the specific vehicle, as some manufacturers have known issues with relay connector corrosion.

Can I keep driving with U356F?

It is generally safe to continue driving with this code if the vehicle runs normally, but avoid long trips or heavy loads until the fault is repaired. If the vehicle enters limp mode or shows performance loss, seek professional diagnosis promptly.

Tools that help

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