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U353F High Voltage System Interlock Circuit N Low

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U353F covers a loop N value under the permitted minimum. The defect can sit inside a consumer rather than in the harness. For the Cummins battery electric systems BES CM2450 EV110B and EV111B, bulletin TSB250044 ties fault codes 8174 (interlock level too low or grounded) and 8175 to the electric air compressor: once the interlock contacts of the HV cable to the compressor pass, the measurement moves to the interlock pins of its low-voltage connector, and above 2 ohms (1 ohm in the first issue of February 2025) the compressor gets replaced. No maker documents U353F. While it is set, the vehicle is likely to lose high-voltage readiness.

Likely causes

Interlock path inside an HV consumer such as an electric air compressor open or high-resistance
HV cable plug on loop N not reconnected properly
Loop N wire shorted to ground

Symptoms

  • High voltage switches off and stays off
  • Interlock fault returns although all cable plugs test fine
  • Auxiliary drive such as the air compressor not running

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–6,000
Time1–6 h

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

Parts involved

hvil loop nhvil wiring

Related codes

U353DU353EU3540U353BP0A0C

References: static.nhtsa.gov, static.nhtsa.gov, law.cornell.edu.