Network, generic (extended)

U356E Anode Drain Valve Heater Relay Control Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U356E refers to the relay that powers the heater on the anode drain valve, more precisely to its coil drive from the fuel cell controller. Drain valves sit in the wettest corner of the hydrogen loop and can freeze shut after a cold night. Hyundai fits PTC heaters to its purge valves, fed by the battery and switched on when a start below freezing is detected; Parker Hannifin places a PTC element right at the seat of a fuel cell drain valve, and Hilite sells drain valves with built-in heating that still open at minus 40 °C. If the relay cannot be driven, the heater stays off, or stays on and drains the battery. In mild weather nothing may be noticeable; in frost an iced drain valve can delay or prevent the start. No maker documents U356E publicly, and its MIL handling is unknown.

Likely causes

Open, short or corrosion in the relay coil wiring or connector
Heater relay faulty
Fuse or supply of the relay circuit missing
Relay driver in the fuel cell control unit faulty

Symptoms

  • Fuel cell warning message
  • Delayed or failed start in frost
  • Often no noticeable effect in warm weather

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–600
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

anode drain heater relayrelay wiring

Related codes

U356FU356AU356B

References: patents.google.com, patents.google.com, hilite.com.