02 Suburban Climate control problem

This is fitted with the dual zone automatic climate control at thre front.

The system knows the outside air temprature and yet does not respond correctly to the controls.

At 90 degrees or above it blows heat (in auto mode at full blast) without stopping.

At 89 degrees it goes into full cool mode. There is no limit to how cold it will take the temperature.

A few questions arise,

1) can anyone help with how to resest the system - i have pulled the fuse but to no avail
2) can anyone tell me how to get the system into self-daignosis mode (i believ holding 2 buttons down would get it to self-check) and then advise on error codes
3) explain where the temperature sensors are and how it piscks up the temperatures for the passenger and driver sides please
4) If it is the maion control unit is it something relatively easy to pull. I am a competent mechanic so I would just need to work out how to get to the scres etc. But are there any cautions I should observe?
5) Does anoyine have access to servise bulletins (TSB) - there is one listed on ALLDATA as 04-01-39-007 Jul 04 that has a title about Climate controls losing temeprature control - that may be it,

many thanks in advance,

of course would love to hear from anyone with this problem thats gooten it fixed

Julian from the UK

You need a body/HAVC/capable scanner to get codes.
There is several TSB's on it.
One has to do with bad wiring.

I would suggest a All data on line sub for all tsb,s and repair procedures.
24.95 US $ for a year online sub. with automatic updates on TSB's.

http://www.alldata.com/products/diy/index.html

And as always make the system has a full charge in it.