This is a tough one. It could be the ECC unit itself... or something simple, like the in dash temp sensor being clogged. Do the recirc and AC lights ever blink after start up? When mine did that, the temp sensor was clogged with dust and the heat was different on either side of the car.
To clean the temp sensor (the right most switch button on the dash with a small grill at the bottom of it - just to the left of the radio) you need to remove the radio. Don't unplug the radio unless you remember where you keep the code! Then reach behind the ECC unit and pull it out, is is just held in with spring clips and can be a little balky :) Once the ECC is out, slide the radio back in.
Reach behind the grill opening for the temp sensor. A long round plastic tube which has pressure fittings at 12 and 6 o'clock. Push these in and pull out the twmp sensor, very carefully! There is a very fragile tube sticking out of the front which cannot be bent! Unplug it from the wiring. Clean off the fan, I used a round tooth pick, very gently, and blew the dust out. Then be ever so careful and put it back in the dash. Shove the ECC back in and try it out.

45 minutes the first time. 15 minutes the second time.

Klaus
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1975 164 w/174,800mi (Sold) 1995 850T w/91,000mi, 1998 V70R w/129,000mi