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Why is My BMW AC Blowing Warm Air?

As we drive through the peak July heat of Summer, one question I’ve been hearing a lot from my subscribers is about your BMW air conditioning system issues. There are three really common issues across all BMW’s with respect to inoperative or weak AC problems:

  1. The center control knurled stratification wheel knob is poorly adjusted
  2. The electric auxiliary fan behind the front bumper has failed
  3. The AC freon leaked out and needs a recharge of 134a

The first place to start diagnosis is the easiest: check your center control knurled stratification wheel knob in the center of the dashboard. Every BMW since 1980-something has one. And it may be the most misunderstood HVAC control knob of any German car out there – even today!

Here’s a background on how this HVAC control knob works and why it could be causing you to have warm air blowing in the Summer and air too cool blowing in the Autumn and Winter.

The often overlooked stratification knurled wheel for the center vent is often the cause of heating and cooling issues. These are erroneously adjusted one way or the other until the season changes, and you forget to adjust it properly for the season.

Before taking your car in for a look at deeper HVAC issues as described in the list above, be sure to check this adjustment first.

Another extremely common reason why AC may not be cold is due to a failed auxiliary electric fan. The electric fan is responsible for pulling cool air through the air conditioning system’s condensor which is really just a mini-radiator for the AC system. When this fan fails, you may notice AC is cold only when driving at speed; this is because air flow from driving is cooling the system. When you come to a stop or idle, the AC air blows warm again. This is when the electric fan would normally kick in to aid in cooling, but since it has failed, it is not pulling in that air.

The E53, E46 and E39 generation are notorious for auxiliary or aux fan failures. Fortunately the fans are relatively easy to replace as a DIY and can be found under $200. Here is a DIY replacement I did last Summer on my E39:

Finally, the third most common reason for warm AC symptoms is due to a leak or component failure. Sometimes a system will have a micro leak that has allowed freon to leave the system. If you’re low on freon or it is empty, the air will blow warm 100% of the time. To start, you can use an inexpensive 134a kit to refill your refrigerant and begin diagnosis. It’s important to use one with a gauge and to never overfill the system beyond BMW’s pressure specifications dependent on your model.

Here is a source for 134a BMW AC refill kits:

Questions or comments? Leave them below or reach me via instagram @BIMMERZEIT.

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