Engine Coolant Level Sensor/Switch Circuit

Possible causes

• CommonCoolant genuinely low, from a leak or from being burnt
• CommonFailed coolant level sensor or float
• CommonCorroded connector at the expansion tank
• PossibleWiring damage
• PossibleLoss of reference voltage or earth
• PossibleSensor coated with deposits from old or wrong-specification coolant

What you might notice

• Coolant level warning
• Engine running hot
• Heater blowing cool
• Sweet smell of coolant
• Visible coolant loss or a puddle under the car
• Check engine light on

What to check, in order

1. Check the actual coolant level when the engine is cold. Never open a hot cooling system - it is pressurised and above its normal boiling point, and opening it sprays boiling coolant out.
2. Look for leaks: under the car, around hoses and the water pump, and inside the cabin footwell where a heater matrix leak shows as a damp carpet and a misted screen.
3. Inspect the sensor connector at the expansion tank for corrosion.
4. Check the reference voltage and earth at the sensor.
5. If coolant is disappearing with no visible leak, have a head gasket test done. Coolant escaping into the combustion chambers leaves no puddle.