Some clarification please.

My FBH runs at 5 degrees or below which is correct.
When the car is fully warmed up and outside temperature still below 5 degrees it still runs.

Reading RAVE, the internal FBH ECU should detect the coolant temperature and then control the FBH as required.

Do I have a fault, can the ECU be interrogated ?
If this is working as designed then I feel a modification coming on !!

Rave suggests that there is a sensor in the heat exchanger that is used to measure coolant temperature - see extract:

The heat exchanger transfers heat generated by combustion to the coolant. A
sensor in the heat exchanger provides the ECU with an input of heat exchanger casing temperature, which the ECU
relates to coolant temperature and uses to control system operation. The temperature settings in the ECU are
calibrated to compensate for the difference between coolant temperature and the heat exchanger casing temperature
detected by the sensor. Typically, as the coolant temperature increases, the coolant will be approximately 7 °C (12.6
°F) hotter than the temperature detected by the sensor; as the coolant temperature decreases, the coolant will be
approximately 2 °C (3.6 °F) cooler than the temperature detected by the sensor.
Rave suggests that there is a sensor in the heat exchanger that is used to measure coolant temperature.

So - either the sensor is not working or the coolant temperature doesn't make the required figure??

No diagrams show this sensor - only the front bumper outdoor temp one.

Investigation will continue with a temperature check of a fully warmed up car where the FBH still operates.

Don't know how relevant it is but the fbh on my l322 cuts on and off when the engine is up to temp.
I've assumed it's to keep at operating temp while idling, keeping emissions low (assume fbh emissions don't count)

andyfreelandy said:

So - either the sensor is not working or the coolant temperature doesn't make the required figure??

Click to expand...

thanks - or it is working correctly and keeping the coolant up to temperature !! Not had it with any other TD4 and FBH so not sure.

I'd run the live data for the engine, to make sure it's getting up above 80°C. IIRC the FBH ECM will try to maintain a temperature of 75°C, so the heater will be more effective. Once the coolant is above 75°C, the FBH should shutdown, or at least stop burning fuel. If the coolant doesn't go above the FBH control temperature, then the FBH will run when it's not needed.

Nodge68 said:

If the coolant doesn't go above the FBH control temperature, then the FBH will run when it's not needed.

Click to expand...

Suggesting that maybe the engine thermostat has failed open??