Thermal runaway- thermistor vs heater vs fan speed

My Taz6 shuts down in the middle of a print with thermal runaway. I watched the hot end temp slowly drop from the 140 degree set temp. until it shut down. My thermistor resistance is 134 k ohms, and the heater cartridge resistance is 20 ohms. Are the fans running too fast, the heater cartridge going bad, or thermistor bad?