U353C

What it means

U353C means the control unit monitoring the high voltage interlock loop, a low-voltage safety circuit that proves HV connectors, covers and service disconnects are fitted, saw circuit 'M' reading higher than expected. This is usually a short to voltage or a wiring fault in that loop, and it can also come from a fault in the monitoring unit. Which components sit in circuit M depends on the model and system, so check the manufacturer wiring information.

Symptoms

• Hybrid or EV system warning light or high voltage fault message on the dash
• Vehicle will not go into READY mode or will not start
• Reduced power or limp mode, or the high voltage system shutting down while driving
• Charging will not start or stops part-way
• Other communication or high voltage related codes stored alongside

Possible causes

• Damaged, corroded or loose connector in the interlock loop (HV battery, inverter, compressor, heater or charger connectors)likely
• Interlock wiring chafed or shorted to a voltage supplylikely
• High voltage connector, cover or service disconnect not fully seated after earlier workpossible
• Water or contamination in a connector causing a false signalpossible
• Fault in a component's internal interlock bridge, for example in the inverter, charger or compressorpossible
• Fault in the control unit that monitors the interlock, or outdated software where the manufacturer documents it as a causerare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting any other high voltage or communication faults and the conditions when U353C set
2. Ask whether any high voltage or underbody work, a collision repair or a charging issue happened before the fault appeared
3. Have a suitably trained technician isolate the high voltage system following the manufacturer's procedure before any hands-on inspection of HV components
4. Visually inspect covers, the service disconnect and the interlock connectors for damage, bent pins, corrosion, water or poor seating
5. Using the wiring diagram for the model, check circuit M for short to voltage, chafing and connector pin tension
6. Measure the interlock loop signal and supply voltage against manufacturer specifications, and check each component's interlock bridge
7. Clear the code, then retest and confirm it does not return, checking the monitoring unit's power, earth and software status only after the wiring is proven good

How to fix

• Reseat and secure any loose HV connector, cover or service disconnect, then retest
• Clean or replace corroded or damaged interlock connectors and terminals, and dry out any moisture ingress
• Repair or replace chafed or shorted interlock wiring, and secure it clear of sharp edges
• Replace any HV component whose internal interlock bridge is proven faulty
• Replace or update the monitoring control unit only if it is confirmed faulty or the manufacturer lists an outdated calibration as the cause

Can I keep driving?

Nearby codes

• U353BGenericHigh Voltage System Interlock Circuit M Low
• U353DGenericHigh Voltage System Interlock Circuit N