The simple answer is---USUALLY, the Phone type COMM cables have a problem. THOSE are NOT Dometic supplied cables but built and supplied by the RV maker. Upper control boards and the 12 button Tstat RARELY are the problem . It is the COMM cables. Look for corrosion on the COMM cable connections at all 3 boards. Do you have LO profile Penguin units or regular Brisk Air units? The usual testing is done at the Board that supplies the wall tstat. You disconnect the Zone 2 and 3 cables from that board and see if just THAT unit functions. IF NOT, then the problem is between that control board and the wall tstat COMM cable. IF it functions to spec, you then connect Zone 2 to the main control board and do a reset and see if Zones 1 ands 2 show up and BOTH function. IF NOT, the problem is the COMM cable from the Zone 1 to the Zone 2. IF all functions do the same for COMM Zone 3 cable and test after reset to see if all 3 zones come up. You use process of elimination to determine where the problem is located. I have test cables to jump between the 3 zones and the wall tstat to verify the tstat and all 3 boards function. Then do what I outlined to find the bad COMM cable. Doug
