What does it mean?
P1624 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Thermostat Control Circuit Malfunction .
Symptoms
- MIL (check engine lamp) on
- Coolant temperature running below normal, particularly at steady motorway speed
- Slow warm-up and weak cabin heating in cold weather
- Higher fuel consumption while the engine stays in its cold running strategy
- Temperature gauge dropping when driving and rising again in traffic
- Cooling fan behaviour changing or fans running earlier than expected
Likely causes
- Open or shorted control circuit between the module and the thermostat heater element
- Thermostat heater element open internally
- Corroded or coolant-contaminated connector at the thermostat housing
- Blown fuse or missing supply to the thermostat heater circuit
- Harness chafed or heat damaged where it runs close to the engine and exhaust
- A plain mechanical thermostat fitted in place of the controlled unit at a previous repair
- Module output driver failure
How to fix it
- Watch coolant temperature in live data through a full warm-up and confirm it reaches and holds the normal operating value
- Command the thermostat heater with the scan tool output test and check that current is drawn
- Unplug the thermostat connector and measure the heater element resistance against specification
- Check for supply voltage at the connector and test continuity of the control wire back to the module
- Inspect the connector for coolant seepage, corrosion and heat damage to the loom
- Replace the thermostat if the heater element is open, then refill and bleed the cooling system properly
- Address the harness or the module driver only once the component has tested good