Range rover 4.0se 1998

Little warning indicator came on the main display (book looking thing), got
the ODBII kit out came up with ecu error code P1179

P1179 AMFR Max Negative or O2 Sensor Signal Circuit Slow Switching From Rich
to Lean (Bank 2 Sensor 1)

I had changed the 02 sensors down stream only 12 months ago.

The rangie was not showing any of the signs of before i.e.. pulling back,
all but cutting out, so I have just carried on using as normal.

Now here's the "STRANGE thing"  on the way home my fuel light came on, so I
popped into Tesco filled the tank, drove off, guess what the error code
signal has now disappeared.

Can it be the octane rating of the fuel I brought from Texaco previously was
not correct for the fuel air mixture ratio?
Or do Tesco have additives take instantly cleaned up my emissions?

Any one else find this or is just a fluke?