Run the defroster on high and bring the vehicle up to temp. If it is still the same then the most likely failure is the water valve unit. The water valve is a pulse width modulated component that is controlled by the heater control panel. There are two feed from it to two sections of the heater core. On your vehicle you have a dual section

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My FSR finally gave in at 15y/140K, so I took the opportunity to take some photos for those who need to trouble-shooting HVAC Blower Motor and FSR circuit. In 90% of the cases, the FSR fails. In 10% of the cases, the HVAC panel fails. There are 2 ways an FSR can fail: 1. Not working at all (this is my case) 2.
Replace your radiator outlet temperature sensor quickly with this guide. Jim from Austin - 2004 BMW E60, N62, GA6HP26Z, Dynamic Drive, Sport Pak, Logic 7, Bilstein B8's, Ti Silver * 2009 BMW E63 M6, Sapphire Black (all options) * 1990 Pontiac Firebird Formula, T-Tops, 305 TPI, Maui Blue.
I'm not sure about your Bimmer, but many diesel (non-commercial) engines bypass coolant past the thermostat to make it to the heater core. If the thermostat isn't flowing once heated up, or not flowing enough, the core will never warm up enough to give off heat.

With fan on high and grounded to the door hinge, I am seeing 12 volts on the green and yellow with greeen strip. The small blue wire is at 8 volts. The brown and tan wires are both

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bmw e90 heater fan not working