Engine management V40 2.0T 2002

[Wood22]

The engine management light comes on for a few days then off, then on again etc, for the last few weeks. It has been plugged into a diagnostic machine (with light on), with the result being, air fuel deviation, does anybody know what this might mean?
The only thing done to the engine, was that my brother borrowed my lamber sensor 2 weeks before the light 1st came on, to see if his was faulty in his S40.
(90000miles)

[MadeInJapan]

It is that sensor (I assume you're talking about the front 02 sensor- Lambda and not lambar, rignt?) that measure fuel/air mixture...maybe he didn't plug it in good or it has some grease on the connectors causing an intermittent problem. See if you can get him back over to help you check it out and clean connectors and check for frayed wires. If this isn't it, you could have a vacuum hose leak that could create the same scenario.