A high-voltage connector that is not fully latched or has backed out slightly on circuit D; Corrosion, water intrusion, or pin tension loss at the interlock connector for circuit D; A damaged or chafed interlock wire on circuit D, often from routing near a sharp edge or a pinch point; A failed interlock switch inside a high-voltage component such as the battery pack, inverter, or motor; A faulty high-voltage control module or battery energy control module that misreads the interlock signal; Aftermarket or improper repair work that disturbed the interlock circuit and left it improperly reassembled

DTC codes / U35xx · Network communication / U351A

U351AHigh Voltage System Interlock Circuit D Performance

Q: What does code U351A mean?

High Voltage System Interlock Circuit D Performance. U351A indicates that the high-voltage interlock loop, a safety circuit that confirms all high-voltage connectors are properly seated, has detected a performance fault specifically on circuit D. The vehicle's control module monitors the interlock circuit for expected voltage or continuity and sets this code when the signal on circuit D falls outside the manufacturer's specified range for too long.

U351A indicates that the high-voltage interlock loop, a safety circuit that confirms all high-voltage connectors are properly seated, has detected a performance fault specifically on circuit D. The vehicle's control module monitors the interlock circuit for expected voltage or continuity and sets this code when the signal on circuit D falls outside the manufacturer's specified range for too long.

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
The vehicle may refuse to start or enter 'Ready' mode, leaving you with a no-crank or no-drive condition
Reduced power or a limp-home mode if the vehicle does allow driving
Often no symptoms besides the warning light, especially if the fault is intermittent
Possible inability to engage the charging session on plug-in hybrids or electric vehicles
Other high-voltage system codes stored alongside U351A in the same module

Common causes, most likely first

A high-voltage connector that is not fully latched or has backed out slightly on circuit D
Corrosion, water intrusion, or pin tension loss at the interlock connector for circuit D
A damaged or chafed interlock wire on circuit D, often from routing near a sharp edge or a pinch point
A failed interlock switch inside a high-voltage component such as the battery pack, inverter, or motor
A faulty high-voltage control module or battery energy control module that misreads the interlock signal
Aftermarket or improper repair work that disturbed the interlock circuit and left it improperly reassembled

How to diagnose and fix it

Inspect all high-voltage connectors for full engagement and confirm the interlock pins on circuit D are not bent or pushed back
Check for water intrusion, corrosion, or debris in the connector housing and clean or dry as needed
Perform a continuity and voltage-drop test on circuit D from the control module to the interlock switch, following the manufacturer's service information
Wiggle-test the harness and connectors while monitoring interlock circuit data to catch an intermittent fault
If the circuit tests good, substitute a known-good interlock switch or component only after confirming the module is not misreporting
Clear the code and verify the repair by cycling the ignition and confirming the interlock status reads normal in live data
If the fault returns, follow the manufacturer's pinpoint test for circuit D, which may require a high-voltage qualified technician

Interlock faults that fool good technicians

A connector that looks fully seated can still have a backed-out interlock pin on circuit D, so a visual check alone is not enough and pin drag must be verified.
The interlock loop is often a low-voltage safety circuit, so a code for circuit D does not automatically mean the high-voltage cables themselves are damaged or that the battery pack is faulty.
Water intrusion from a clogged drain or a missing connector seal commonly sets interlock performance codes long before any high-voltage insulation fault appears.
After a collision repair or battery service, a partially latched connector is the most frequent cause, and the code may not set until the vehicle is driven or charged.
Some vehicles set U351A when the 12-volt battery is weak because the module misreads the interlock voltage reference, so always verify system voltage before condemning high-voltage parts.

Can I keep driving with U351A?

Do not continue driving with an active high-voltage interlock fault, because the vehicle may disable the high-voltage system without warning and leave you stranded or unable to restart. Have the vehicle inspected by a qualified high-voltage technician before attempting further use.

Tools that help

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