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Hello, thank you for using JA. I will need as little more information before I can give you an answer. So, let me list a few things and see if you can get me the information. First, what equipment are you connecting it to ? as wire terminals mean different things with different equipment. Next, I need the model to the old thermostat MPLS is not the model so if you can locate that or take a photo this will make it easier to get you the correct information. Also, please clarify the models on you question it reads that you tried a CT87N but want to use a CT31A for a replacement. Thank you Richard
Thank you the picture was worth a 100 words, this sub base uses the B as a heating connection. So what ype of equipment do you have hooked up? often this goes to a heating damper motor. Depending on what is connected to B will tell us if you an use it or not.
For future reference you have a T87 thermostat with a Q539A sub base
Hello, just checking back into to see if you found out the information? Depending on what is hooked up we might be able to get this thermostat to work.