I have a frontera 2.2 (A) which would not produce any heat in the cabin. Found out I had a cracked head, got a reconditioned one fitted. Have changed the pipes round to the inlet to the heater matrix and changed the thermostat. When I did this run the engine to get the coolant round and great heat in the cabin, not over heating, fantastic....... Finished topping up the coolant, went out this evening and no heat! Driving home, heat again! Not great but heat and it only seems to give me heat when traveling slow, in a low gear, and high revs!