Incorrect Temperature Reading

The problem i'm having is that the temperature reading is constantly 7.4 degrees celcius. The temperature of the room is about 21 degrees celsius. Even when the printer is heating the reading stays constant. I recently changed the thermistor and heater cartridge so i know that those aren't the problem. The reason I changed the thermistor is that it was originally reading 7.4 degrees celsius. I recently changed the firmware to repetier .92. It was working great for 10-15 prints, but then one of the prints lifted off the bed and everything went haywire. I thought that the thermistor went bad because the printer heated extremely hot, and the plastic started to smoke. I wasn't home to see what happened. Any suggestions would be greatly appreciated.