We had some subzero weather while I was on vacation. My 2004 Jaguar was parked outside for several weeks. We had another very cold, icy day, and heater did not work. Fan works, rear window defroster works, but no hot air blowing at high setting. What could the problem be? Is this something we can fix? Also driver door will not open sometimes in cold or heat First noticed the problem when it was hot. Had a part replaced and was working fine. Then we had some extreme cold and experienced the same problem. Your thoughts?

What is happening here is that the faster the heater blows air through the fan, the more it cools the liquid in the heater core, and so heat output is reduced. The root of the problem is either the engine is not warming up enough, or the heater core, and/or heater core lines, are plugged up, not allowing the water to circulate properly in the heater core. Here is what I would try.

1. Top off the radiator and overflow.
2. Is the engine warming up properly? Is your temperature gauge showing normal temperatures? If it is not, then replace the thermostat.
3. If steps 1 and 2 do not solve the problem, get the vehicle in a shop, locate the heater core lines, take them apart and flush out the heater core. Look for blockages in the fittings. Do a flow test on the core. It may be clogged and need to be replaced.

Heater problem first:
You say no hot air at high setting. Is that no heat or no air blowing?