A thermistor is a device having a negative coefficient of resistance (ohms) vs temperature (Celsuis).
When cold the thermistor is high resistance(ohms), low resistance (ohms) when hot.
If you are showing a red light, the ECU is probably reading a low resistance from the sensor circuit , please check the garage has installed the sensor correctly, I think the sensor is short circuited, a trapped wire or the connector damaged or mis-wired.
You mention the mpg gauge is blank, a long shot, but do they share a common line or feed in the wiring diagram? If you disconnect the sensor, does the mpg display return to normal?