Hi all,
It should be noted that there are 2 temperature sensors in the system, air temp and engine/oil temp. Both sensors are identical ( same NTC thermistor ). The sensor itself is unlikely to fail. What is most likely is that due to corrosion or wire break, the effective resistance will increase ( colder ) or be infinite ( bloody cold ).
Failure of either sensor is likely to cause the over rich mixture.
The best check to make is to check the resistance of the sensor from the ECU connector.
If I'm not mistaken the sensors come in on pins 4 and 18 of the 15M connector - http://www.cajinnovations.com/MyECU/pics/My15MMkIII_PCB.png
Measure resistance to ground on pins 22,23 or 24. These are usually connected together in the ECU but with the connector off, you will need to see which of these to check. Try Carl's schematics.
The resistance to expect is here - http://www.cajinnovations.com/MyECU/temperature.htm but the easest check is for a cold engine both the sensors should read the same