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DTC codes / U35xx · Network communication / U351B

U351BHigh Voltage System Interlock Circuit D Low

U351B indicates that the high-voltage system interlock circuit D is reporting a voltage level that is lower than the manufacturer's expected range, which the computer detects by comparing the signal on that circuit against its calibrated threshold. The interlock is a safety loop that confirms high-voltage connectors and covers are properly seated before the vehicle allows the traction battery to energize.

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

Symptoms you may notice

  • Illumination of the high-voltage system warning light or a master caution indicator on the dashboard
  • The vehicle may refuse to enter 'Ready' mode or may not start at all
  • Reduced power or a forced limp-home mode if the vehicle does drive
  • The high-voltage contactors may open, cutting off the traction battery and stranding the vehicle
  • Often no symptoms besides the warning light if the fault is intermittent or only affects a redundant interlock branch

Common causes, most likely first

  1. A high-voltage connector that is not fully latched, allowing the interlock pin to lose contact
  2. Corrosion, water intrusion, or terminal fretting inside an interlock connector
  3. A damaged or pinched interlock wire in the harness between the connector and the control module
  4. A faulty interlock switch or jumper inside a high-voltage component cover or service disconnect
  5. A poor ground or reference voltage supply to the interlock circuit at the control module
  6. An internal fault in the module that monitors the interlock loop, such as a failed pull-up resistor or driver

How to diagnose and fix it

  1. Inspect all high-voltage connectors and service disconnects for full engagement and secure latch position
  2. Check the interlock circuit wiring for continuity, chafing, and corrosion between the connector and the monitoring module
  3. Verify the reference voltage and ground at the interlock circuit terminals with a multimeter, following the manufacturer's pinout
  4. Test the interlock switch or jumper in each high-voltage cover and service disconnect for correct continuity when closed
  5. Clear the code and perform a road test to confirm whether the fault is intermittent or hard
  6. If the circuit checks good, substitute a known-good monitoring module or perform a guided diagnostic with the factory scan tool
  7. Repair or replace the faulty connector, wire, switch, or module and re-verify that the interlock loop reads within specification

Interlock faults that mimic component failures

  • A partially latched high-voltage connector is the most common cause of this code, and the latch can look engaged while the interlock pin is still not making contact.
  • Technicians often replace an expensive high-voltage component when the real fault is a corroded interlock terminal inside the connector housing.
  • The interlock circuit is a low-voltage safety loop, so a low-voltage reading does not mean the high-voltage battery itself is weak or failing.
  • Water intrusion after a car wash or heavy rain can set this code intermittently, and the connector may dry out and clear the fault on its own.
  • Some vehicles use a redundant interlock path, so a single open branch may set U351B without preventing the vehicle from starting.

Can I keep driving with U351B?

Do not continue driving if the vehicle will not enter Ready mode or if the high-voltage warning is active, because the contactors may open without warning and leave you stranded. Have the interlock circuit inspected by a qualified high-voltage technician before attempting further travel.

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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