How can I diagnose this symptom?

I have recently noticed that the heater blows cold air when on auto mode. If I set the control to hi it will blow good heat. The blower regulates well and all vents are selectable by desired function.
It seems that when in auto mode and the control is set at a set temp the system thinks then cabin is warm inside. I thought first of low coolant or an open thermostat. The engine temp fluctuates between 78-85C by cluster guage which is normal for this car.

Any Ideas?

thanks

With car cold and the key in position 2, press and hold the REST button for >5 seconds until the display changes to a 1 on the left side. Write down the 1 and the number on the right. Press the left auto button to advance to 2 and write those down. Continue until you get back to 1. Also note the ambient temperature at the time of the test.

I'm particularly interested in the reading at #1, which is the inside temperature sensor reading, but the rest may be important if that one is correct. Also reading trouble codes may help, but that's slightly more involved.

I will do it this afternoon and post findings

greatly appreciated

Results of the control module test. Ambient temp was 50 F. Are these normal readings?

Thanks again
1 18
2 11
3 30
4 30
5 26
6 81
7 04
8 22
9- 05
10 0.0
11 1.7
12 4.3
20 0.0
21 00
22 00
23 00
24 11.4
40 68
41 19
42 00
43 144

Did you ever solve this problem? My 99 E320 does the same thing.

still blowin cold when not on high

I've had mine in to the mechanics twice. They changed the thermostat (no go) and they said the next step is probably to change the control face. Just wondering if anyone else has has the same problem.

There is a DIY on rebuilding a valve that is relay controlled for the split climate control. If you look under the hood bside the ecm interface boxyoull see two sylinoid devices that open and close to let more or less water to the heater core. I am sure thay play a big role in regulating the heat per side.
Im sure it is under climate control/mono valve. I just looked it up

Climate Control: