So that is the one close to the engine isnt it, with this coming on only during the mot at high revs would this suggest its missreading during normal conditions causing part or all the problems and during high revs it over rich to an extent that is then registering out of limits then bringing the light on. Im aware there could be more than one issue maybe but i hink a service, oil and plugs and a sensor would be a good start
Cheers