Bed preheat error  

RE: Bed preheat error

the Thermistor is a temperature dependant resistor, 
100Kohms is the expected temperature at 25 degrees C
If your ambient temperature is different to 25C, then it's reasonable that the resistance is different to 100K

If the thermistor was short circuit you would get a MAXTEMP bed error
If the Thermistor was Open Circuit you would get a MINTEMP Bed Error

Printers with the Spiral wrap protection for the heatbed wiring  were sometimes plagued with the thermistor wires being pulled, during the frequent bending of the Spiral wrap, during printing. 
A number of people have found that the thermistor has been pulled out from the adhesive tape, under the heatbed. 
It may be that your thermistor is not in good contact with the heatbed. 
It's certainly worth checking that the thermistor bead is near the centre of the silver foil tape and kapton tape patches, near the centre of the build plate. 
If I recall correctly the thermistor is normally just to the left hand side of the centre of the build plate... 
(The centre of the build plate, is occupied by fixing screws for the Y axis)

I Believe it's always worthwhile having spares on hand, I usually keep a heatbed thermistor, extruder thermistor and a heater on hand... 

One thing to bear in mind, is that the wiring to you heatbed also flexes a lot. 
Early Mk2 printers had the heatbed wires soldered onto the heatbed, which caused a potential work fatigue point, right at the back of the heatbed
Mine fractured right at the end of the solder joint... 
I Have replaced my heatbed wires with flexible silicone wire I used 16AWG, some people say 14 AWG would perhaps be better... 
check that you have a nylon filament support in the wiring bundle, and that the heatbed end is well secured... 
On my Mk1 heatbed, prusa used ABS Filament for the support. the ABS fatigued quite quickly, fractured, and then concentrated the flexing of the heatbed wiring, on a very short section of the heatbed wiring, and caused it to break

Later Mk2 heatbeds had crimp on terminals and nuts and bolts to hold them in place, these may have come loose which can cause a similar effect. 
if you have nut and bolt connectors the washer MUST NOT be between the connector and the heatbed! 

All Mk2 printers had plugs and sockets between the Mini Rambo board and the heatbed wires, these sometimes become loose, and they can overheat. If this has happened, make sure you replace them with named brand connectors... there are chinese knock off versions which may be  substandard

regards Joan

so... check your thermistor location, make sure the thermistor is firmly held in place with the tape, and check yout heatbed heater wiring and connectors