The duovalve controls the amount of engine coolant thatís allowed to pass through the two heater cores.
The duovalve is located on passenger's side of the engine compartment, just in front of the fuse box. Itís easy to spot as it has two small cylindrical aluminum cylinders. There are 5 Torx screws (t-10?) holding it in place. With the car COLD, the torx screws can be removed. A magnetic drive will ensure one doesnít drop and lose a screw. The electrical connection should be left in place. After the screws are removed, gently remove the aluminum cap of the duovalve - pulling straight up. A thin/flexible putty knife can be used to GENTLY loosen it. That will expose two plungers in the lower portion of the valve. If these are sticking open, it will allow hot coolant to flow through the heater core. This impairs the air conditioningís cooling capacity. Gently, the plungers can be removed. After drying and cleaning the plungers with a clean rag and a safe chemical solvent, the lower portions can be lubricated with a non-water soluble grease (some people use Vaseline). Afterwards, itís ready for reassembly.