Expert: John Kingery - 9/20/2010

Question
Heat comes out on the front passenger side of a 2003 BMW 325i when the air conditioning is turned on.

Answer
When you have two zone A/C, the warmer of the two is achieved by opening that side of the heater core.

If heat is coming out one side, it is likely to be caused by either the control valve for that side of the heater core to be stuck open, or that valve is being told to open by a bad signal coming from the controller.  First guess would be the former.

Also, are you certain it is heat, and not just no A/C on that side?  I believe these have a single evaporator, so if cold on one side should be cold on both.  However a flap may be stuck causing ambient air to come in rather than A/C.

If it's the former problem with the heater core valve, starting out with a cold engine should result in getting cold A/C from both sides, and the heat should come only after the engine heats up.  This is another test you could run.