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U3512 High Voltage System Interlock Circuit B Performance

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U3512 is stored when interlock loop B still conducts but behaves implausibly: its resistance drifts outside the permitted window, the returned signal does not follow the test pattern in time, or two readings of the same loop disagree. CATL's interlock patent explains why loops misreport: contact impedance between HV parts, supply fluctuations and static discharge can cause false alarms. Samsung SDI points out that different loops may have different detection and reaction times, so loop B can be judged by other rules than loop A. Oxidised pins, a connector not fully latched or a harness rubbing on the body are likely causes, as Mercedes lists for its interlock codes P0A0A00 to P0A0D00. No maker documents U3512 itself. The HV system may drop out repeatedly rather than stay off.

Likely causes

Oxidised interlock pins raising the loop resistance
Connector in loop B seated but not fully latched
Harness rubbing on the body with intermittent contact
Interference or supply ripple distorting the loop signal

Symptoms

  • HV system switches off and on again unexpectedly
  • Ready mode lost while driving or at start
  • Interlock codes in several modules at once

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–1,500
Time1–5 h

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

Parts involved

hvil loop b

Related codes

U3511U3513U3514U3516P0A0B

References: patents.google.com, patents.google.com, static.nhtsa.gov, law.cornell.edu.