Won't stop heating up

When I try to start a print or even swap filament out, the heater begins to heat up, but it will heat well beyond the expected temperature. I measured this with an outside thermometer since the one in software is stuck on about 16% expected temperature. Is this a problem with the thermometer in the printer? I’ve tried extruding some plastic with the current version of the software and some legacy ones and had all the same problems, so I don’t think it’s a software issue. I was previously having issues with the heater not working at all, but then purchased a new extruder unit, which heats up fine. The only thing I can think to do is to cut the thermometer off the old unit and solder it to the new one, but I wanted to see if any of you had another solution before I did that.