You can get an infra-red theremostat at most auto stores for undr $20 and they will come in handy for may things. Point it at the thermostat housing and see what temperaure you get.
There is an oil temperature sensor on the transaxle valve body assembly. The 1 wire temperature sensor is on the thermostat housing. The 1 wire seems to be fore the fan circuit and dash gauge. So, the 3 wire must be for the CPU. I could not find it in any diagram I have, they are not always complete. However, this is generally the setup when 2 coolant temp. Snesors are used. Unless, you have an engine oil temperauter ensor and then that is used for the fan switch and then the other 2 are for the gauge, 1wire sensor) and the ECU, 3 wire sensor). So you might need to test the other 2 sensors or replace them. Checking the thermostat temperature with an infra-red thermoeter against the gauge display is a good check as operating temperature is generally dead senter of most stock gauges.
To, clear the code if it keeps coming back, dis-connect the negative batery cable for 1 minute and that will reseet the ECU.