Network, generic (extended)

U354A Hydrogen Tank A Temperature Sensor 1 Circuit

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U354A reports an electrical fault on the first temperature sensor of hydrogen tank A. Tank sensors matter most while refuelling: compression heats the gas, and FMVSS 308 slows the pressure ramp once the gas inside passes 85 °C; on the Mirai the tank temperature is passed to the dispenser by infrared. No maker publishes U354A, but Honda shows how such faults appear on the 2017 Clarity Fuel Cell under its own codes P1F76 (main tank) and P1F77 (sub tank): they set during fuelling, and because the sensor sits inside the tank, the first remedy in Tech Line article ATS 170903 was a new tank. Bulletin 19-001 later blamed many cases on outdated software in the fuel cell communication unit. Sensor 2 of the same tank is U354B.

Likely causes

Outdated software in the unit that evaluates the tank sensors
Temperature sensor inside the tank failed (often only as tank assembly)
Open or short in the sensor wiring or connector at the tank

Symptoms

  • Code sets while refuelling
  • Hydrogen system warning
  • Refuelling slower or ends before the tank is full

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€50–4,000
Time0.3–6 h

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

Parts involved

h2 tank a temp sensor 1sensor wiring

Related codes

U354BU354CU354EU3549

References: static.nhtsa.gov, static.nhtsa.gov, law.cornell.edu, media.toyota.co.uk.