U353A
High Voltage System Interlock Circuit M Performance.
- 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
- 3A
- FaultThe specific fault in that area
What it means
U353A means the control unit monitoring the high voltage system interlock loop has found that circuit 'M' is not performing as expected. The interlock is a low-voltage safety loop that runs through high voltage connectors, covers and components to confirm they are properly connected. The unit saw a signal outside its expected range, an intermittent signal or a signal at the wrong time. Which parts sit in loop 'M' depends on the model, so check the manufacturer's wiring information.
Symptoms
- Hybrid or EV system warning light or message on the dashboard
- Car will not go into READY mode or will not start
- Loss of drive power or limp-home mode
- High voltage battery will not charge, or charging stops
- Fault appears intermittently, often over bumps or when the car is hot or cold
Possible causes
- High voltage connector or service disconnect not fully seated, or its interlock pins bent, corroded or contaminatedlikely
- Damaged, chafed or corroded interlock wiring or low-voltage connector in the looplikely
- Recent high voltage work where a cover, connector or interlock link was left loose or fitted wronglypossible
- Water ingress or contamination at a high voltage component connector, such as the inverter, charger, compressor or battery packpossible
- Faulty interlock switch or internal interlock circuit in a high voltage componentpossible
- Fault in the monitoring control unit, or outdated software where the manufacturer lists it as a known causerare
How to diagnose
- Read all stored and pending codes plus freeze frame data, and note whether the fault is current or intermittent and what other codes are present.
- Look up the manufacturer's information for which components and connectors are in interlock circuit 'M' on this model and engine or drive system.
- Carry out a visual inspection of the accessible high voltage covers, connectors and the service disconnect for correct fitment, damage, water or corrosion. Do not touch orange high voltage cabling or open high voltage components unless you are trained and the system is isolated.
- Inspect the low-voltage interlock wiring and connector pins for chafing, bent or pushed-back terminals, corrosion and poor tension.
- Wiggle-test the interlock harness with the ignition on while watching live data or the fault status, to find any intermittent connection.
- Once a trained technician has isolated the high voltage system and verified it is dead, test the interlock loop for continuity and resistance against the manufacturer's specification.
- Check for any software updates or technical bulletins that cover this code on this model.
How to fix
- Refit and securely latch any loose high voltage connector, cover or service disconnect, following the manufacturer's procedure.
- Repair or replace damaged interlock wiring and connectors, and clean or replace corroded terminals.
- Dry out, seal and repair any water ingress, and replace any connector or component that is contaminated or damaged.
- Replace the faulty component or interlock switch once testing proves it is the cause.
- Apply a manufacturer software update only if an outdated calibration is a documented cause for this model.
Can I keep driving?
Do not keep driving if the car warns of a high voltage fault, loses power or will not start, as the high voltage system may shut down without warning. High voltage checks and repairs must only be done by a suitably trained technician.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.