I have checked my plug and can confirm that my GS is definitely running on the rich side.
I've been reading a lot about O2 sensors and it's all very interesting...
Apparently the business-end of the sensor is a sintered porous ceramic which allows O2 to diffuse though it to make contact with a pair of electrodes which produce a potential difference somewhere between 0V and 1V, depending upon the level of O2, and this voltage instructs the ECU to increase or decrease the fuel mixture. It makes sense to me that the sensor will have a finite life because its function will depend upon the porosity of the ceramic which will become increasingly blocked by exhaust deposits. While these sensors are supposed to last 100,000+ miles we all know about the bath tub failure curve don't we?
There are numerous videos on Youtube claiming that we have been convinced / hoodwinked / brainwashed into believeing that O2 sensors fail completely and need to be replaced, rather than regularly cleaned - possibly becauase it's a quick and easy fix - although this is relatively easy process. These videos O2 sensors being soaked overnight in petrol, thinners, carb cleaner etc and then residual deposits being removed by an ultrasonic cleaner. A general recommendation is that they should be cleaned every 10-15k kmiles, ie at every service. This makes sense to me because the porous ceramic will eventually become blocked with all the carbon and whatever deposits in the exhaust, and that regular cleaning should slow down the blocking process.
As it looks as if my O2 sensor is not working properly I will try cleaning and report back my findings.
Has anybody else any experience of this matter to add to the thread?