Just bought a 2002 Mondeo TDCI but when driving the car back the heaters didn't get warm at all, after about 15 miles I stopped for fuel then when I got back in the car and started it up, the heaters were working great as they should.

After a few more miles the heaters seemed to go cold again and never warmed back up.

Fans are working fine but not the heater, anyone have any ideas?