Can the temperature control unit be replaced ? Is it a OEM part?
2003 discovery 11

 

Yep, pretty much everything is an OEM part, but dealer may want to sell you the whole seat. Can be bad module, or the heat sensor for it in the driver's seat could be bad. Least costly would be buy a seat at a salvage yard and gut it for parts. My yard charges like $22 - $25 for any seat. Dismantlers on this forum may be a source, they might have a heavily ripped seat that needs to be parted out.

See attached from RAVE manual, if you like what you see the whole manual set download is free.