I found somewhere that holding in BOTH knobs on an auto a/c will make it go into some kind of diagnostic mode. My questions are:

1) Can you CHANGE anything?
2) Are these codes of any use to a DIY'er? If so, where would one find a listing of the codes and what they mean?

There is nothing secret about it. But you definitely won't find it in the owner's manual.



On-Board Diagnosis Mode
The CJ2 air conditioning is equipped with an on-board diagnostic feature which allows the heater-A/C control to detect certain system faults. The diagnostic feature may also be used to display certain data parameters. If the heater-A/C control detects a fault in the system, the man symbol will flash during the first 2 minutes of vehicle operation.

Entry into the diagnostic mode is gained by pressing in on both temperature knobs at the same time for 5 seconds. The following display modes are found within diagnostics:

The diagnostic pointer
The diagnostic value
By rotating the driver temperature knob one detent in either direction, the display is toggled between pointer and value. The value displayed is the decimal equivalent of the RAM contents. A 2 is formed using the minus sign and the leading 1 in order to display values over 199.

A diagnostic pointer mode is identified by a minus sign preceding the pointer number. The pointer provides an index, which identifies the corresponding diagnostic value. The diagnostic trouble codes (DTCs) reside under diagnostic pointer -00. The pointer will be set to this position upon initial entry into diagnostics. If more than one code exists, the codes will be displayed sequentially at a rate of 2 seconds per fault. Rotating the passenger temperature knob will switch the diagnostic pointers. A wrap around feature is provided if the pointer is continued past either end. Other data used by the computer resides under different diagnostic pointers, and may not be of use in observing the system. Press any mode button in order to exit the diagnostic mode.


Important
A 2-digit trouble code indicates the fault is current. A 3-digit trouble code indicates a history fault.


Clearing Codes
Press the A/C mode button in order to clear codes from the heater-A/C control.

Any of the following components may cause malfunction in the temperature controls:

The refrigerant system
The electrical wiring connections
The vacuum connections
Faulty components that have mechanical problems, such as sticking valves
As a part of the diagnostic process inspect the following components:

The refrigerant, vacuum or mechanical systems. Refer to Vacuum Test .
The blower controls. Refer to 66-0 .
The compressor controls. Refer to Cell 67: RPO L36, L67 .
The air delivery controls. Refer to Power and Grounds .
The temperature controls. Refer to Electrical Diagnosis. Complete the diagnostic procedure for the heater A/C control microprocessor.



Make sure to turn the climate control off first, then hit both buttons at the exact same time. I know one time it took it like 5 seconds, the next time it took like 15.Then rotate the left knob until the desired number appears.

Position Description
-00 Error codes Click here for the Trouble Codes chart

-01 (servo position) 103-107 43-162
-02 (servo position) 128 120-202
-03 (servo position) 150 162-194
-04 CJ2 Fan Speed Low=15, 29, 44, 58, 71, 86, 110=High, CJ2 Unit will adjust to other increments as needed
-05 (servo position) 93 88-191
-06 (servo position) -152 00-252
-07 (servo position) 00 0, 34-255
-08 (servo position) -155 00-255
-09 (servo position) 00 0, 23-255
-10 ? 04 04
-11 CJ2 Vent Mode 1=Def, 2=Bilevel, 3=Auto, 4=Lower, 5=Def+Lower
-12 Engine Coolant Temp -60 degrees 80=104, 95=131, 100=140 Peak@135°F
-13 Actual Vehicle Speed in MPH
-14 (servo position) 00 21=0
-15 ? 39 44
-16 Radiator low fan turn on temp -106 207
-17 Radiator low fan turn off temp 196 199
-18 ? 50 51
-19 (servo position) 162-169 40-172
-20 Affected by cabin temp sensor 89 50-172
-21 (servo position) 00 ?
-22 Sunload Sensor -117 LIGHT=126, 218=DARK
To return to normal operation, press the MODE button next to the Passenger temperature knob.


This is a list of the Trouble Codes displayed by the CJ2 Unit.
A two digit code indicates a current fault while a three digit code indicates a historic fault.
Trouble Code Description
00 No System Fault
01 or 101 Inside Air Temperature Sensor Short
02 or 102 Inside Air Temperature Sensor Open
03 or 103 Ambient Outside Temperature Sensor Short
04 or 104 Ambient Outside Temerature Sensor Open
05 or 105 LH Electric Actuator Open or Short
06 or 106 RH Electric Actuator Open or Short
07 or 107 UART Serial Data Line Fault
08 or 108 Solar Sensor Open

Nice write-up Bob, of course now my head hurts from trying to take it all in.

That's why I put the word "secret" in quotations marks the first time. Maybe the codes aren't actually secret, but it's certainly not common knowledge or listed in an OTC repair guide -- or even Alldata (not that I could find, anyway).

Thanks. You've provided EVERYTHING a person could want to know when it comes to A/C unit diagnostic codes...

Sorry -- channeling Ed McMahon from the TS old days...

That's why I put the word "secret" in quotations marks the first time. Maybe the codes aren't actually secret, but it's certainly not common knowledge or listed in an OTC repair guide -- or even Alldata (not that I could find, anyway).

Thanks. You've provided EVERYTHING a person could want to know when it comes to A/C unit diagnostic codes...

Sorry -- channeling Ed McMahon from the TS old days...

All Data or mitchell and the factory service manual list the codes.
Has it got a problem? Or what is it doing or not doing?
If it has a code usually you will get a blinking light or a indication of a problem.
And should show a code on the diagnostic check.
If not working right one of the first checks would be low freon.
MT

All Data or mitchell and the factory service manual list the codes.
Has it got a problem? Or what is it doing or not doing?
If it has a code usually you will get a blinking light or a indication of a problem.
And should show a code on the diagnostic check.
If not working right one of the first checks would be low freon.
MT

Evidentally you are not familiar with a Grand Prx CJ2 digital climate control system. When there is a problem the maintenance man icon will turn on or blink.

And yes the procedure is in the GM service manual but its free here and what is posted is self explanatory.

This may seem like a REALLY dumb question but that climate control diagnostic feature wouldnt happen to be able to clear a SES light would it?

This may seem like a REALLY dumb question but that climate control diagnostic feature wouldnt happen to be able to clear a SES light would it?

Sorry Mike561 but no. You'll need an odb-ii scanner.

What problem are you having? Make a new thread/post, if necessary.

just the evap code that keeps triggering it off whenever i have it scanned.

For those asking -- I asked the orignal question simply out of curiosity -- nothing more. But, thanks for the willingness to help.