I found this over on the US sister forum that gives a little bit of info about it being the SAIR system that adds air into the exhaust manifold
https://www.jaguarforums.com/forum/...-15/lean-both-banks-i-am-about-give-up-91415/
Thanks for the link ....
I am guessing this is pointing me in the right direction.
With that in mind I found this on the internet - not specific to Jaguar's but more cars in general.
As emission control strategies grew more sophisticated and effective, the amount of unburned and partially burned fuel in the exhaust stream shrank, and particularly when the
catalytic converter was introduced, the function of secondary air injection shifted. Rather than being a primary emission control device, the secondary air injection system was adapted to support the efficient function of the catalytic converter. The original air injection point became known as the upstream injection point. When the catalytic converter is cold, air injected at the upstream point burns with the deliberately rich exhaust so as to bring the catalyst up to
operating temperature quickly. Once the catalyst is warm, air is injected to the downstream location - the catalytic converter itself - to assist with catalysis of unburned hydrocarbons.