ACC I CODES 3A, 3U
The following is the factory training manual test procedure for code 3A,3U in the ACC I variant.
Remove the ashtray, reach in and push out the ACC control panel. With the key off, unplug its connector. With an ohmmeter, check resistance across pins 4 and 25. You should have 2000 -17000 ohms with the ACC set to LO. You should have 790-2000 ohms with it set to high(allow temp stabilization time after moving temp setting and turn key off each time before plugging/unplugging the control panel to take resistance readings
Now for my experiences with this code. I like to try and find for myself why things fail and long ago found that the solder on the thin wires joining them to the pins is weak. After I discovered this, I have been able to successfully repair all the ones with this code. The resistors wiring is broken and you will get an open circuit reading from pins 4 and 25. Remove the glove box and the sensor plugs into the air box in the center of the car on the right side lower aspect of the air distribution box. Doing the resistance test from the control panel's connector will verify the wiring as well as the sensor. If you get an open, first check the wires for continuity(never seen a problem here though). Unplug the sensor and check each wire for resistance between its ends. It has two wires, violet and black on its connector. Extract the sensor from the air box and unplug it. Open up its white plastic case(held with interlocking tabs) and look at its thin wires, usually one of them has come unsoldered. You can carefully resolder it and restore its function. I have fixed many this way. I have no idea what a new one costs, I've never had to replace one, just fixed the old ones. You might also want to strengthen the other solder joint there with a little more solder so it doesn't let loose in the future while you have it out. Takes about 20 minutes start to finish. Put everything back together and rerun the self diagnostics, code should be gone