Just got back from an MOT with a fail due to a lambda reading of 1.08. I understand that this means that the engine is apparently running slightly lean with too much air. What are the likely reasons for this, and are there any simple fixes, injector cleaning, stifling the air intake etc?

 

A small hole or leak on the exhaust can cause this

 

How about a new o2 sensor?

 

That's what I'm trying to avoid really, but that thought did cross my mind, I wonder if a knock following my engine rebuild could have done this...

If there was a hole in the exhaust is this likely to be after the Lambda sensor and cat or before, I'm struggling a little to figure this out.

 

OK, so this has now been resolved after much investigations and many monies.

The probe wasn't far enough up the exhaust.

So it was getting the additional air readings from the atmosphere, probably due to the tips being 3" and i guess potential for some backdraught perhaps. Either way, all is well now.

Mods; delete if you want, I'm sorry for wasting valuable forum time...