My father has the 98 E300 I drove back from Phoenix last year. AC was ice cold. Since AC season started my dad says the AC works great and blows ice cold. A couple of days ago he called and said it blows cold for a while then it seems like it blows hot air and the amount of air decreased like the blower decreased. I went by and looked at it today and took by SDS with me. When I started it up it blows ice cold out the proper vents. I connected the C4 to the car and went thru getting it connected and starting HHTWIN. By this time I notice its not blowing cold and is blowing out the defrost and side vents, nothing coming out of center vents. I start to suspect maybe the heater duovalve is not closing the flow of coolant. Checking the sensor data on the PBU (before I got HHTWIN connected) showed the heater core temps were high indicating it was heating. Once I got into adaptations in HHT I commanded the valve to turn off and on. It definitely seemed to turn the valve off and on as commanded. I then went into activating the different air flaps. It really did not seem to change the air flow and directed. It always seemed to stay in defrost. I started to suspect a vacuum leak also. One other thing, the aux fan never came on and I figured it should have. I commanded it to come on in activations and it did not. ACC was set for 71 on both sides. I then lowered temp to low. A couple of minutes later it seemed like it was cooling like it turned off the flow to the heater core and the air flaps actually cycled and started blowing out the center vents like it should. A minute or 2 later it went back to only blowing out of defrost. Since it did this I really do not think it has a vacuum leak. I am wondering if the PBU is defective? He is supposed to bring it out here tomorrow and I will swap the PBU out with one I have on the shelf and see what happens. I will also need to apply power to the aux fan to see if it kicks on. Has anyone ever seen or heard of one of these actually going bad?