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DTC codes / U35xx · Network communication / U351C

U351CHigh Voltage System Interlock Circuit D High

U351C indicates that the high-voltage system interlock circuit D is reporting a voltage higher than the manufacturer's expected range, which the computer detects as an abnormal signal on that specific interlock loop. The interlock is a safety circuit that confirms high-voltage connectors and covers are properly seated before the vehicle allows high-voltage operation.

SystemNetwork
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaNetwork communication

Symptoms you may notice

  • High-voltage system warning light or service vehicle soon message on the dashboard
  • Vehicle may refuse to start or enter ready mode, leaving you with a no-start condition
  • Reduced power or limp mode if the vehicle does drive
  • High-voltage system may be disabled, preventing charging on plug-in models
  • Often no symptoms besides the warning light if the fault is intermittent

Common causes, most likely first

  1. Damaged or chafed interlock wiring harness on circuit D, causing a short to voltage
  2. Poor connection or corrosion at a high-voltage connector associated with circuit D
  3. Faulty high-voltage interlock switch or connector position sensor on circuit D
  4. Water intrusion into a high-voltage connector or component housing
  5. Internal fault in a high-voltage component module that supplies the interlock signal
  6. Aftermarket or improper repair that introduced a voltage source onto the interlock circuit

How to diagnose and fix it

  1. Scan all modules for related codes and record freeze-frame data to understand when the high voltage was present
  2. Inspect the high-voltage interlock circuit D wiring and connectors for damage, corrosion, or loose pins
  3. Measure voltage on the interlock circuit D at the connector with a high-impedance meter and compare to specification
  4. Disconnect and reseat the relevant high-voltage connectors, checking for water intrusion or debris
  5. Test the interlock switch or sensor on circuit D for proper continuity and voltage output
  6. If the circuit checks good, diagnose the high-voltage component module that supplies the interlock signal
  7. Clear codes and perform a road test to confirm the repair, verifying the interlock circuit reads normal voltage

Interlock Circuit D Voltage Traps

  • A short to voltage on the interlock circuit is often caused by a pinched wire near a high-voltage connector, not by a failed high-voltage component.
  • Technicians sometimes replace expensive high-voltage components when the real fault is a corroded connector pin on circuit D.
  • The interlock circuit is a low-voltage safety loop, so a high-voltage reading on it usually points to a wiring fault rather than a high-voltage battery issue.
  • Intermittent U351C codes often appear after a repair where a harness was disturbed, so check recent work before condemning parts.
  • Some vehicles set this code if the 12-volt system voltage is unstable, so verify battery and charging system health first.

Can I keep driving with U351C?

Do not continue driving with an active U351C code because the high-voltage system may be disabled or operating unsafely. Have the vehicle inspected by a qualified high-voltage technician before attempting further driving.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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