Should the timed climate ,heat/defrost below 0 . Set mine but nothing happens,and manual heat / manual vent is not lit on the menu screen ……..2012 Westminster 4.4 tdv8

Yes it should trigger when ambient temperature is low ( below 8 degrees C I think ). It can be switched on manually from the touch screen menu, programmed on a timer from the touch screen menu or triggered remotely with a dedicated key fob. If the battery voltage or fuel level is too low it won’t fire up or, if operating, will shut down.

I seem to remember that to trigger it manually from the touch screen you have to switch on the infotainment with the ignition off. The menu option should then appear highlighted rather than greyed out.

It’s a well researched topic on here so try searching FBH or Auxiliary Heater to find the threads.

IMHO it’s worth persevering to get the FBH working as they’re a real boon in the type of weather we have currently. TDV8 4.4 2012

Just tried with ignition off.
Lit up for 2 seconds ,and auto light lit up on the heater control panel.
Then nothing.
3 quarter tank of fuel

I used to get this. Do you hear the clicking of the FBH fuel pump start?

If not, it's likely the pump has failed and the FBH ECU is shutting it all down due to no fuel supply. 2015 Range Rover Autobiography 4.4 SDV8

Or if the battery is getting old it will cut out shortly after fire up. I have this currently as battery is 5 years old and due a swap out Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
2025MY Defender D350 90 in Silicon Silver on coils
1974 Series 3 Lightweight = "Millie"
Many, many other Landies over the years
My preferred specialist: www.glenrands.co.uk
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On the MY 10-12, the FBH will provide cabin heat <15 degrees C
At -20 C, the heat goes to the engine block

Have you checked the fuse? Mine used to fire up for 5 seconds then turn off. Found the fuse missing so replaced and now works like a dream.

Could be the coolant pump on the FBH that's given up or has a corroded contact or such.