L322 Supercharged Lambda Sensor MOT Troubles

Hi, After another year of fantastic trouble free motoring in our 2007 4.2 Supercharged Rangie it was time for the MOT this morning. It passed but only just, with a couple of advisories for front brake discs and pads, lower track control arm and the rear brake pipes needing to be done it really struggled with the emissions test. We do occasionally have the engine management light pop on which indicates a problem with the 2nd (downstream) sensor but has no noticeable effect on the running of the car and after resetting the fault it might not come on again for several weeks. We initially had a CO of 0.6+ which is miles over the permitted maximum of 0.2 incidentally the engine management light was on. So I cleared the fault and blasted it round the block and just managed to get it into the 0.2 upper limit.
So I need to get at least one new sensor and I've looked at the various online fitting guides and will probably get a Denso sensor. They list 2, DOX-0413 and DOX-0109 which is a universal sensor and is cheaper but is it an inferior product why list 2?
With the vehicles mileage now approaching 130k should I change all 4? Anyone know the life expectancy of a Lambda sensor or do they work perfectly until they fail or do they decay.
I have noticed the fuel economy has worsened recently with 25mpg easily possible I am now lucky to get 19 -20.
Never having had to fit Lambdas on any previous vehicle I'm a little in the dark so any advice would be very welcome. Thanks Ian

Plenty of duck oil or wd40 on the sensor beforehand, not sure if it's 27mm? You can get a special socket for removing the sensor. Chances are it'll be well seized into the bolt hole!! But if it's coming out you can cut the top of it and tackle it with a normal socket (size depending) and tighten the new one with a good spanner. Don't forget a bit of copper slip just incase you need to do it again in the future.
As for changing all four, that's your call and your budget. A good sensor isn't cheap and there is four???
A cheap sensor may well rear it's ugly and a good sensor might outlive your ownership.. that is if you don't keep it for another ten years...

I may be wrong but I don’t think the downstream sensors contribute much. Is that the only fault you are getting and what diagnostics are you using ?
Different engine but I have had downstream sensor faults on mine for about 3 years with no ill effects and my emissions on my MOTs are like butterfly farts

Yes I thought the downstream sensors only checked the catalyst performance. The fault comes up as an under voltage condition or missing signal, stupidly I forgot to copy down the exact fault code. It's just a cheap code reader and resetting device but seems to do the job. We had an issue over Christmas with having to wade through a flooded road and I'm wondering whether the catalyst has been damaged with being quenched. I'll get the sensor changed and investigate from there.

The range rovers being so complicated tend to respond better to the more advanced land rover specific diagnostic equipment as you probably already know.
i have tried really cheap readers and they offer very little in the way of an accurate diagnosis when you compare the same fault condition with something like the faultmate or allcomms.

i think the icarsoft is quite a good relatively cheap one but something like the GAP equipment seems popular with the later models like yours.

Wading can cause all sorts of problems and even something like an abs sensor can influence all sorts of other systems but probably not the engine.
Some decent diagnostics with live data could help with tracing your emissions and fuel consumption issue if you can pick one up or borrow one.
keep us posted on how you get on