Based on what you are saying I agree it sounds like the temperature control unit. Is it possible to swap the unit with the one from machine B? It is interesting that two same brand unit have the identical issue. If not, then you may want to check the Vent valve on the control unit to see if it is clogged up. That's a fairly common failure but more often then not it will stick open and not heat properly. My next thought which is easy to confirm is to check the pump rotation and make sure it's going the right direction. Let us know what you find.