The only thing that changes the temperature at vent level seems to be outside temperature. This morning it was 30F outside, driver vent blew some warm air but passenger vent blew cold. This afternoon while it was 54F out both blew warm while on interstate. Last week when it was 20F both blew cold.

New.Water Pump not working. I have a 2008 328i and replaced the water pump and thermostat. The pump still doesn't work. I checked the fuse in the right side of the engine compartment, and the 2 fuses under the dash. Plus the fuses over the battery. All is ok. I have 12 volts at the pump in 2 places, and checked ground connection at the pump.

The resistor pack is a module for a set of series resitors that BMW uses in several older models and the diagnosis for failure was the fan would only blow on the highest setting. The E90 blower has a "motor contoller", P/N 64119222072, that retail is $115, or $92 at Tischer BMW. The blower motor is closer to $180 retail.
Remove plug from fan and run 12v live from battery to equivalent live on fan . Run earth from fan earth to an earth point in engine bay. It should run. If it does then it's likely to be the fan switch. This will be found on most cars either in radiator or one of its feed pipes.
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A technician will diagnose your BMW 320i heater using the symptoms you report as a starting point. Because your BMW 320i heater is a complex mechanism that uses an electric fan to blow air through a heater core that is heated by the engine coolant, the problem could be electrical, mechanical, coolant-related, or a combination of these systems.

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bmw e90 heater fan not working