Fix Erratic heat and cooling problem on BMW
BMW climate control units relay on input from two temperature sensors. The most common failure (aside from failure of the entire control unit which is quite high) is the ambient (outdoor) temperature sensor. The sensor is located in an air duct under the front bumper. BMW puts it there to isolate it from hot air coming off the radiator or A/C condenser. However, that under-bumper area is always hitting curbs. Plastic pieces often break off from this area, and when they do, they rip out your ambient temp sensor with. So check that area for damage.
Next, check the interior temperature sensor. It’s located right in the control unit and often gets clogged with dust and dirt. There’s a small fan inside the unit that draws in interior air. The dust and dirt insulate the sensor and it provides incorrect readings. Try removing the faceplate and vacuuming out the dust. If you spray it with canned air, you’ll just lodge more of the dust inside the unit.
© 2012 Rick Muscoplat