W140 - Climate control won't give DTCs

[AdamT]

I just replaced my climate control unit in my '94 S320. It seems to work fine, except I can't run the check for DTCs. I had this problem with my previous climate control unit, but figured it was due to many of its buttons no longer functioning, including the rest button.

Unfortunately, I'm guessing the problem with this is me, since I have been unable to make either unit show the DTCs using the directions i've found online.

Here's what I've been doing:
- put left temp knob at the highest setting, the right at the lowest
- turn the ignition to the accessories-on position (have also tried with car running)
- press left AUTO button
- immediately press REST and 0 at the same time and hold them

Nothing happens when i do this . When I do it with the car off, but ignition in accessories position, the temp displays do go blank for just a second (and there's what sounds like a vacuum unit popping), but then they just come back on.

Before replacing the unit, I was having troubles with the unit locking up -- the climate control would not change settings or turn off until the battery was disconnected (even when the car was turned off). It has also been switching off on its own very frequently (sometimes as often as every minute or two). You then must turn it back on to have heat.

I'm not sure how to tackle these troubles without the DTCs. Do any of you guys have experience with this?

also, I'm replacing the duovalve this afternoon. I don't see how that would help the electronic troubles, but at least I won't be frying all the time.

Thanks in advance.