[RMM321]

2015 F10 528i with dual zone automatic HVAC. We had no issues with the HVAC to date (Heat or A/C). This might also apply to other BMWs with dual zone climate control, I know it does for our E91.

When driving up to DC through VA in cold snowy weather (24-28 degrees outside) my wife noticed the passenger side of the car was only blowing cold air. Drivers side was nice and toasty, nothing but cold on hers. As a temporary fix we kept the air on re-circ, closed her vents and put a blanket in front of the floor vent, the heat from drivers side kept it decent.

I suspected a blend motor was not actuating. Once I got home and put on the code reader there were no codes for the HVAC and I was able to activate the blend motors through the scanner. Not a blend motor.

I did not have any sweet smells in the cabin, there was no loss of coolant.

That trip we drove in just about every normal driving situation you can experience. We did just shy of 2,000 miles at highway speeds, surface city streets, and in crawling DC traffic jams. We had air temps from 10 degrees one morning to 80 degrees a week later in DC. If we were losing coolant, it would have been noticed.

Time for the forums, I found a few articles saying it was a blend door actuator, a failed HVAC ecu, clogged heater core, bad water pump, etc.

None seemed to describe what we were experiencing.

I found an E90 article talking about a failed secondary coolant pump on the heater control valve failing (our E91 wagon has this secondary pump, our F10 does not). In that article they described that the heater core has two separate zones and the heater control valve can send coolant to the separate zones independently based on commands from the HVAC control module.

In our case it seems the control valve for passenger side heater core was stuck closed. Thus, no hot coolant being sent to the passenger side of the car.

A new control valve was $67 (there are kits to replace the seals that can degrade and cause the plunger to stick, for less than $70 bucks for brand new, I didn't want to do that).

It was a very easy fix, the hardest part was draining/vacuum filling the coolant. The control valve had 3 connections for coolant hoses (1 in, 2 out) and an electrical connector. It is mounted on the left front wheel well in the engine bay and just popped on and off with rubber grommets. There weren't any screws or bolts holding it in place.

We immediately got full function back. It went back to effortlessly switching between Heat or A/C in both zones and independent of each other.

Obviously, if you smell sweetness in the car or you notice you're missing coolant there may be other causes. Also, if there's a code logged for a blend motor then that's probably a more likely solution.

I wanted to share in the F10 forum to give a possible other suggestion if others had similar frustration of trying to figure out what is going on.

I've also heard that the valves can get stuck open where you get nothing but heat from one side of the car, no matter what your HVAC setting is.