I don’t know why it would do that besides the motherboard being dead… or the power module on the board being dead…
have you changed the settings in the MakerBot to change it to duel extruder… or if the sensor in the wrong port…you may be able to try the port for a second extruder on the board if it is available… and change the setting to MakerBot 2x and see if the board is dead… when you start a print douse it show the goal temp?.. i don’t know what else to say…
Are you doing a large print? Sometimes when I’m doing a print about the size of the build plate, I have to manually adjust the heat to keep it from going too low.
Also I have found that taping cardboard around the rep 2 keeps in the heat when I’m doing a slow print.
But without more details as to what your actual problem is I’m just guessing.
The Replicator 2 only has a thermocouple which you said you already changed. Besides, since you said it is measuring the temperature and the temperature is about ambient but the temperature is not changing, then the problem is almost certainly related to the heating circuit. Of the heating components your choices really are only the heating cartridge, the wiring or the mother board. The wiring and heater cartridge are easy to check by measuring their resistance as mentioned before. Actually the power supply could possibly be another problem but that is also fairly easy to check by measuring the output voltage(s). If the voltages are correct then you should be getting at least some heating,even if the power supply is not putting out enough current to reach the correct temperature but you are not seeing any heating, so if the voltage is correct and the resistance is correct across the heater and wires then by the process of elimination the problem is the motherboard.
It should be fairly low, although I don’t know the exact number, I would be surprised if it were greater than 20 ohms. Certainly, if the resistance is over 1,000 ohms, it is way too high! Yes, it is possible to fix the motherboard if you know what you are doing and have decent soldering tools and skills, especially doing SMD reworking. However, I am halfway decent at SMD soldering and I flubbed my mother board and ended up buying a new one. (I may still to fix the original as a spare). Of course, first you need to be good enough at component level troubleshooting to find the part(s) that need replacing.
I had the same problem with a Makerbot 2. Had an issue with the hot end where PLA was leaking out over the top of the extruder print block and down over the print nozzle. Took it all apart and cleaned it all up but it still leaked so replaced the print block, nozzle and barrier tube and checked it all before reassembling and everything heated up fine. Then when I tried to run a print the the extruder wouldn’t go above ambient temperature. it doesn’t display and print errors, the thermocouple is clearly working as it registers a temperature on the display which can be raised by simply holding it your hand. The cartridge heater is relatively new also. Doesn’t appear to be any voltage going to the heater when i checked under the bot on the motherboard - presume its the motherboard thats shot. Any ideas people?