Thanks, Keith, I see what you mean. If there actually is an issue of over-fuelling then any fault certainly betrays no detectable odour and the car is currently garaged. But about 10 days ago I did a journey of around 150 miles and about midway I noticed the MIL had come on – this only ever happened previously last year when the catalyst was seized by some opportunists, though, seemingly resolved following fitting of the (non-Toyota) cat & lamda sensor* replacement.
I then tried to investigate further with the OBD2 code-reader. This is a pretty new item for me and with my ‘investigation’ I found that there was a fault code of P1035. Unintentionally, I must have managed to erase the code from the vehicle. Of course I am aware that this will hardly remedy the problem and it now is apparent that there is a more pressing issue to resolve than just the fan running constantly. Maybe, as you say, should the long term fuel trim rise to 25 then the MIL will illuminate again? Meanwhile are there any recommended checks to make given the ‘clues’? I will take a shuftie of the MAP sensor and look for any leaks etc.
* I have heard that when replacing a lamda sensor the OEM type (Denso) should be used. This was not the case but the replacement item would appear to have worked perfectly well for many months.