[fixherz4]

Car: 2003 BMW Z4 2.5i with no warm/hot air blowing through heater, only cold air. The Z4 has Automatic Heat & AC with temp gauge on knob.

There is no Hot air being blown. The heater/AC Fan blows as expected. Adjustment for directional air to upper, middle and lower works as expected except on cold air. Engine heats up properly, temperature gauge working correctly. Turning thermostat to highest doesn't make a difference.

Issue started when the coolant overflow box was replaced. Replaced unit due to crack in overflow box. Flushed system at time Multiple times. Coolant wasn't dirty at the time of flushing.

Question: does the thermostat direct hot water to heater core or is it automatically pushed/sent through the core ? Is there mechanism which precludes hot water to travel through heater core?

I am about to replace the Automatic Heater/AC unit.

I repair the car myself when possible.

Suggestions.