Network, generic (extended)

U3556 Hydrogen Tank C Valve Control Circuit

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U3556 is the control circuit code for the valve on tank C, the third tank found only on three-tank layouts such as the Nexo or the current Mirai. On the Mirai the two crosswise tanks behind the long central one are the smaller ones, so losing one costs only part of the range. That makes the fault easy to overlook while driving. The controller recognises an electrical fault in the valve drive directly and stores U3556, whereas a valve that sticks mechanically only shows up as missing hydrogen, which Hyundai's patent finds by comparing supply with consumption. Check coil, connector and driver first. No manufacturer documents U3556 publicly. Tank C stays shut; expect a warning and less range. The enable path of the same valve is U3557.

Likely causes

Wiring or connector fault at the tank C valve
Valve coil open or shorted
Driver stage for valve C faulty

Symptoms

  • Hydrogen system warning
  • Range noticeably shorter after a full fill

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€100–5,000
Time1–8 h

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

Parts involved

h2 tank c valvevalve wiring

Related codes

U3557U3558U3550U3553

References: newsroom.toyota.eu, patents.google.com, en.wikipedia.org.