The thermistor is used to sense the temperature of the heat sink. The firmware in the drive/inverter monitors this constantly and the checks for steady state readings. When it sees a steady state reading it figures the thermistor has failed.
If you're competent enough to wire in a pot to fool the drive you should be able to do the following:
1) Be sure the heat sink is as clean as you can get it using compressed air FROM A DISTANCE. Don't use 120 psi shop air from a home made nozzle an inch away from the circuit board. You can blast other stuff off the circuit board that you really need. Or create a short circuit without realizing it.
2) Be sure all the power semiconductors are firmly seated against the heat sink. If any are loose you need to verify there isn't any dust, grit, etc. between the semiconductor and the heat sink. If the heat sink compound (white stuff) has dried out you need to remove the old and replace it. If you remove any semiconductor from the heat sink you need to be sure you get it mounted flat against the heat sink during re-assembly and USE FRESH HEATSINK COMPOUND.
3) Check the cooling fan for proper speed. The bearings could be bad and the fan won't turn fast enough to keep the heat sink cooled. If it's questionable then replace it. www.jameco.com has a wide selection of fans. Don't worry so much about brand name, etc. Just match the voltage, physical size AND flow direction. They are cheap enough to have a spare and one day the mfg may decide to discontinue that version.
4) Replace the thermistor. You might have better luck finding a replacement at Newark | US - Electronic Components Distributor | Electronic Parts Distributor, or Mouser Electronics. Use the same guidelines as when replacing semiconductors. While the thermistor isn't generating heat it needs to sense it accurately. Keep it clean, and firmly seated against the heat sink.
Be careful, use your head, and DON'T FORCE ANYTHING to move. The semiconductor(s) may be adhered to the heatsink with nothing more than surface tension - which is GOOD and means there's nothing wrong there any nothing further needs to be done - but a mis-placed screw driver used as a pry bar can crack the semiconductor case and ruin it. Just be damn sure there isn't a screw holding it that you've missed.
WC