U3523
High Voltage System Interlock Circuit F Low.
- Generic
- Network
- Generic code
- High severity
How this code is built
- U
- SystemNetwork
- 3
- Defined bySAE, the same on every car
- 5
- AreaNetwork area 5
- 23
- FaultThe specific fault in that area
What it means
U3523 means the control unit monitoring the high voltage system interlock loop has seen the signal on interlock circuit F reading low. This is lower than expected, which suggests a short to ground or a fault in the loop. The interlock loop is a safety circuit that confirms HV connectors, covers and service plugs are fitted, so the system may be shut down to protect people. Which components sit on circuit F depends on the make, model and HV system layout.
Symptoms
- Hybrid or EV warning light, or a high voltage system fault message on the dash
- Car will not go into Ready mode or will not start
- Loss of drive power or limp mode while driving
- HV battery contactors open and the car cuts out or will not charge
- Charging will not start or stops early
- Other HV or communication fault codes stored at the same time
Possible causes
- Interlock circuit wiring chafed or shorted to ground, or damaged by water ingresslikely
- Damaged, contaminated or corroded pins in an HV connector or interlock terminal on circuit Flikely
- Faulty interlock switch or loop within an HV component such as the battery pack, inverter, charger or compressorpossible
- HV connector, cover or service disconnect not fully seated after earlier workpossible
- Crash or underbody impact damage to HV cables, connectors or harnesspossible
- Fault inside the control unit that monitors the interlock looprare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting whether other HV, isolation or communication faults are present and when the fault set.
- Check the manufacturer's information for which components and connectors are on interlock circuit F for this model.
- Carry out a visual check of accessible low voltage harness and connectors for damage, chafing, water, corrosion or signs of impact. Do not handle HV components unless trained and qualified.
- Make sure the HV system is properly isolated and proven dead by a qualified technician following the manufacturer's procedure before opening any HV connector or cover.
- Check that HV connectors, covers and the service disconnect are fully seated and their interlock pins are undamaged, clean and not bent.
- With the system isolated, test the interlock circuit F wiring for a short to ground and for correct resistance and continuity, using the wiring diagram.
- Isolate sections of the loop component by component to find which part or harness section pulls the signal low.
- Clear the code after repair and road test, then recheck that it does not return.
How to fix
- Repair or replace the damaged interlock wiring or harness section, and secure and protect it against chafing.
- Clean, repair or replace corroded or damaged connector terminals and fix the source of any water ingress.
- Reseat and properly secure any loose HV connector, cover or service disconnect.
- Replace the HV component whose internal interlock circuit is faulty, as confirmed by testing and manufacturer guidance.
- Replace the monitoring control unit only after the wiring and components have been proven good and the manufacturer advises it.
Can I keep driving?
Do not keep driving with this fault. It involves the high voltage safety system, and the car may lose drive power or shut down without warning. Have it inspected by a technician trained in HV systems.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.