On Friday, 31 January 2020 17:16:03 UTC, Jim GM4DHJ ... wrote:
On 31/01/2020 17:14, Jim GM4DHJ ... wrote:
is it the upstream sensor one or the downstream sensor two that has the
biggest effect on MPG ? .....
...if faulty

Only the upstream is within the fuel control feedback loop. The downstream really just determines if the cat is working or not.