How do you know the thermistors are working? When left off for a while, does the bed temperature reported on the display reflect the ambient room temperature? Does the reported temperature rise quickly as when the bed heat is turned on?
Thermistors change their internal resistance as the temperature changes, but the real question is HOW does the resistance change? There are two general categories of thermistor: those with Positive and those with Negative temperature coefficients. The ones with positive coefficients have their internal resistance increase as the temperature increases. With negative coefficients, the resistance drops as the temperature goes up.
Within each of those categories is the response curve. Does it change linearly with changes in temperature, or logarithmically? You don’t really need to know any of this specifically, just be aware that it’s not enough to just replace a thermistor, you have to replace it with one that has the same temperature coefficient and response curve.
So the question is: did anyone change the thermistor and accidentally use the wrong one?
The second thing that crosses my mind that will be easy to check, is: is the bed thermistor properly attached to the bottom of the bed? While the Hot End (nozzle) has a hole that the thermistor gets inserted into, they are usually just taped to the bottom of the bed. Did it come off? Is it just hanging in air? If it did, that would show the correct ambient temperature after being off for a while, but would not reflect the actual bed temperature if it isn’t properly taped to it.
The thermal runaway issue is the firmware recognising that it has heated the bed for, say, 20 seconds and expected a 20C rise in temperature but only got a 1C increase. It then shuts down.
By the way, the opposite is also true: If the bed heater fails, you’ll still get a thermal “runaway” error, even though the temperature hasn’t gone anywhere. It’s kind of a misnomer in that case.
So, to test:
Have you checked all the cables end-to-end to make sure everything is plugged in correctly and snugly?
Do you have access to a voltmeter? You will need to check that 24V is being applied to the bed, if only briefly, for the printer where the bed doesn’t heat up at all. On the one that heats, as I said, check that the thermistor is reading normal room temperature after it’s been off for a while, then see if it reacts to heat being applied. If yes and no respectively, check that the thermistor is properly attached to the underside of the bed. If no and no respectively, check the wiring.
Ask if anyone has serviced, or even moved the machine recently. Sometimes, while being moved or serviced, a cable can be pulled loose. IT MAY NOT COME COMPLETELY OUT, however.
See this thread for an example of someone with an Ender 6 with a similar problem. On post #15, I posted a photo in which I highlighted a connector that turned out the be the source of his problem. Note how little movement of the connector was required to disconnect power. That’s the kind of thing you’re looking for.
If someone has serviced the machine, and it involves replacing the bed thermistor with, possibly, the wrong one, you can either replace it with the correct one, or select the correct model in the firmware. While, technically, there is no harm in putting any available thermistor in (within reason), models that aren’t compatible with the original factory installed thermistor require a change in firmware. That’s OK until the day comes to update the firmware. The manufacturer’s new firmware will expect the original thermitor model. You have to remember that it’s been changed and edit each and every new firmware to reflect the change. If you forget, you get the behaviour you are describing wherein the firmware expects a thermistor with characteristic X and you have one installed with characteristic Y.
Let us know what you find.