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U3517 High Voltage System Interlock Circuit C Low

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U3517 says the level read back on interlock loop C stays too low. Depending on the design, an opened plug in loop C, a wire chafed to ground or a failed source in the monitoring module all lead to it. In current-fed designs such as the Texas Instruments reference circuit, a short to ground draws extra supply current while an opened loop does not, which helps to separate the two. For loop A, GM uses a 5 V test voltage and a 30 % lower limit, which indicates the margins involved. Mercedes has technicians measure interlock loop resistance segment by segment on model line 243, a method that suits loop C just as well. No manufacturer publishes U3517 itself. Until the fault is cleared, the contactors remain open.

Likely causes

Interlock wire of loop C chafed to ground at a module or the body
Connector in loop C unplugged, sensed as a low signal
Broken interlock connector housing or terminal
Source stage for loop C failed in the monitoring module

Symptoms

  • Vehicle cannot be started
  • HV system warning in the instrument cluster
  • Charging unavailable

Repair estimate

DifficultyHardWorkshop job
Parts and labour€80–1,200
Time0.5–4 h

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

Parts involved

hvil loop chvil wiring

Related codes

U3515U3516U3518U3513P0A0C

References: ti.com, static.nhtsa.gov, gsi.ext.gm.com.