Problem with XL950 Comfortlink II 7” color display – improper indoor temperature display and communication error 126.


I had a Trane 3 ton XL 20 i, TAM 8 air handler, all new duct work, and XL950 Comfortlink II thermostat installed March, 2012. We are located in the Tampa, Fl area, slab floor, single story, air handler in the attic.

The system was fantastic for about 3 months, we loved it, extremely comfortable, electric bill dropped in half, and then it stopped cooling properly just at the start of the peak of Florida's summer.

The installers came out and replaced some sensors that were defective. The installers had been alerted to the sensor issue by Trane and they had replaced some on other systems they installed. Ever since then, I have had issues that the installers are pulling their hair out over. I will address the issues in separate posts, hoping that someone else may have had similar issues and found a fix (the issues relate to the XL950 controller, communication errors, and the system’s lack of cooling).

This post is for the XL950 Comfortlink II 7” color display – improper indoor temperature display and communication error 126.

The XL950 Comfortlink II, 7” color touch panel controller no longer displays the correct indoor temperature. I believe that the XL950 was displaying the correct temp up to the point of the loss of cooling. After that it is way off. Prior to the lack of cooling issue, the display appeared to be fine.

The XL950 is located on a wall in the middle of the house, just a few feet from the main air return, which is ceiling mounted. This is the same location the last digital thermostat was located, which worked fine for many years.

I placed a type K thermocouple connected to a calibrated Omega temperature meter at the same location (almost touching) as the temperature sensor in the XL950 for all comparisons (lower right hand corner- sensor is visible if one looks there). The Omega meter was also used to verify other thermometers used to measure general room temperature.

The XL950 would read 85 F indoor and the type K thermocouple (and various other indoor thermometers) would read 76 F. Then, all of the sudden the XL950 would jump to 83 F and maybe 82 F, but would eventually go back up to 85 F. The temperature differential would be 8 to 12 degrees different between the XL950 and any other temperature reading. This was verified by the AC technicians.

The temperature differential was too great to compensate through XL950 calibration, and occasionally would read within a couple of degrees of the Omega meter. As the day progressed into evening, the XL950 would read closer to the other thermometers. Also, if the system was in stage 1, 50% air flow, the reading was closer. As the system progressed into stage 2 and higher air flows, the readings appeared to be wider apart and the XL950 tended to read in the mid 80’s with room temp actually around 78 or so. With the improper reading, the system would never hit set point, and would run continuously.

A thought was that hot air was being introduced through the drywall or the hole the wires go through as the fan increased speed. To eliminate that, I sealed the hole with insulation and mounted the XL950 on a large piece of high density insulating foam, and even went as far as mounting the XL950 ¾” off of the foam so the XL950 “floated” in air and was not touching anything. We also protected it from any excessive air flow to the return.

Still acted strangely, reading the same is before all the isolation work, so the technicians and I thought it was a defective XL950 or power to it was fluctuating. Checked power and it appears to be solid, well within spec.

Also during this time, the XL950 would randomly display Error code 126, which is related to equipment not matching the setup equipment list or a communication error among equipment. This error would clear itself so we would only see it maybe 1 out of 10 times.

So, thinking the XL950 was defective, the technicians replaced it with a brand new one. They had trouble getting that one to even communicate properly with the system, and when it did, the room temp reading was again wrong, just like the original one.

In the process of attempting to solve this issue, they replaced the wire from the XL950 to the main control board and replaced the main control board. Still no change with either the original or the new XL950.

At that point, they put a T900 thermostat in to look at other issues related to limited cooling (will be described in a separate post).

The T900 has been in for a little over a week and we have not seen any communication errors (have not looked at history – I don’t know how to do it on this T-stat or the air handler), and the room temperature is within a degree or two of the Omega meter.

Does anyone have any suggestions on how to get the XL950 to read correctly?
Anyone aware of any issues with the XL950?
Any thoughts as to what else the installer might check?
The system is under warranty, and I don’t touch it, I only take various temperature readings to supply to the installer, look at t-stat settings, and call the installer.

I really, really like the XL950 and to go to the T900 is like going from a smart phone to an old flip phone.

I have been monitoring this site and learning tremendous amounts over the past 18 months. The amount of knowledge here is fantastic. The knowledge I gained from this site helped me decide what new system to install, and what to look for in the technicians. Thanks to all that have supplied all this valuable information.