Not sure if it is firmware or not?

I have just finished building a printer and it is working fine except for one issue.
When I plug in the thermistor for the heatbed I get this error message scrolling continuously;

Error:
: Extruder switched off. MINTEMP triggered !
Errorrinter halted. kill() called !!
Error:
: Extruder switched off. MINTEMP triggered !
Errorrinter halted. kill() called !!

The thermistor is the same type as used in the hotend, both are 100K
I tried swapping the hotend for the heatbed and also the thermistors too, and I still get the same error message, so I guess that the settings in the firmware may be at fault??

It seems strange that the error message refers to the extruder.

Any help would be appreciated.

PS: I have tried a few Printrboards (revD) and they all do the same with my setup.

We use Marlin and if we get this message, we know right away to look at the thermistor wiring. This is a safety feature built into Marlin to check and see if there is any temp reading from the thermistor. If the firmware gets no reading at all, it assumes the thermistor is broken and shuts down the extruder. That's why you get the error message about the extruder switched off.

To help others wishing to help you, tell us what firmware are you using? Using what program do you see the error message? What are your electronics?

Check your wiring first.

I am using Marlin and also a Printrboard rev D. The host software is Pronterface.
The reason I feel that the problem must be with the settings is because I can take the thermistor that is being used for the hotend that is working OK and plug it into the heatbed thermistor socket, that is when I get the error message. If I plug in the thermistor for the heatbed I still get the same message. Both thermistors are the same type, 100K
I have tried two boards and got the same result from both of them.