P0C99
Hybrid/EV Battery Temperature Sensor L Circuit Low.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- C
- AreaHybrid propulsion
- 99
- FaultThe specific fault in that area
What it means
P0C99 means the hybrid or EV control unit saw the signal from battery temperature sensor 'L' fall below the expected voltage range. This is a circuit low fault, so the reading suggests a short to ground, a faulty sensor or a fault in the sensing circuit. Which physical sensor is 'L' depends on the make, model and battery pack layout.
Symptoms
- Hybrid or EV system warning light on, or a general malfunction light
- Reduced power or limp-mode operation
- Reduced charging or regenerative braking
- Battery cooling fan running constantly or behaving oddly
- Battery temperature reading on live data that is implausibly low, or fixed
- Reduced electric-only range or the car switching to engine-only running on a hybrid
Possible causes
- Faulty battery temperature sensor 'L' reading out of rangelikely
- Sensor signal wire shorted to ground or to another circuit, from chafing or damagelikely
- Damaged, corroded or contaminated connector or terminals at the sensor or battery control unitpossible
- Water or coolant ingress affecting the sensor circuit or pack wiringpossible
- Fault in the battery monitoring unit or its sensor input circuitrare
- Outdated software in the battery control unit where the manufacturer has issued a fix for false readingsrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting any other battery temperature or battery monitor codes.
- Check live data for all battery temperature sensors and compare 'L' with the others and with ambient temperature when the car has been cold-soaked.
- Check for manufacturer service bulletins and software updates relevant to this model and battery pack.
- Have a trained high-voltage technician isolate the system following the manufacturer procedure before any pack-side work. Do not touch orange high-voltage components otherwise.
- Inspect the sensor wiring and connectors for chafing, pinching, corrosion, moisture and bent or pushed-back pins.
- With the system isolated, test the sensor circuit for a short to ground and for continuity, using the wiring diagram for the model.
- Measure the sensor resistance and compare it with the manufacturer specification at a known temperature.
- If the wiring and sensor are good, check the supply and reference voltage at the control unit, and consider the control unit itself.
How to fix
- Replace the faulty battery temperature sensor, following the manufacturer's procedure for the pack.
- Repair or replace the damaged wiring or the section of harness causing the short to ground.
- Clean, repair or replace corroded or damaged connectors and terminals, and seal against moisture.
- Fix the source of any water or coolant ingress and dry or replace affected components.
- Update the battery control unit software if a known fault is documented, or repair or replace the unit if testing confirms it is faulty.
Can I keep driving?
Medium severity
The car may be driven carefully for a short time, but the battery temperature is not being monitored properly, so power or charging may be limited. Book it in soon, and stop driving if the warning escalates, there is a burning smell or the pack feels abnormally hot.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.