While I was printing my printer randomly started letting out a long loud beep. So, I turned it off and on now every time I turn it on it gives that error message and won’t let me do anything else. I tried to replace the thermistor on the hot end but it keeps on showing the error code. I think I have to replace the thermistor on the bed but I’m not sure how to do that.
I presume the bed doesn't feel hot, therefore something is interfering with the printer's ability to sense it's bed temperature.
Two possibilities come to mind:
The bed's temperature sensor is broken/damaged or otherwise returning a false reading. In this case, replacement of that sensor is your only option.
The wiring is damaged between sensor and mainboard. Given the printer is at least 5 years old, that's a lot of flexing on the wire and terminations.
If I were you, I'd turn the printer over, open the mainboard cover, and identify the leads to the bed's thermistor. And the leads to the print-head thermistor. Take a photo, and then disconnect the leads.
Then get a multimeter, set to Ohms, and compare the resistance of each thermistor. They should be around 100 kOhms when at room temperature. If you're getting close to 0 or "many megaohms" then the sensor is faulty and needs replacement. Compare resistance of both thermistors to make sure you're getting a good value and your method is sound.
If the resistance is close to zero then you have a short. That may be through a gap in the wiring, but is more likely a dead sensor.
If the resistance is super high, that could easily be a break in the wire. Pull the wires out and look for sharp bends/kinks. If the wire is faulty, you may get away with replacing the wires to the existing sensor. Do use stranded wire no-thinner than your existing wires.
Good luck !
This may be helpful, but it is talking about a print-head thermistor:
