OK, seen a few older posts on this. Just wondering what the latest knowledge is. My UK-based 1997 Legnum VR4 has failed on CO of 0.7% - all other emissions are fine. It was marginal last year, but not this time. I did a good long drive with some cleaning additive as the last journey before the MOT. The car seemed nice and hot for the MOT itself, and the tester tried it several times. Local garages I've tried say they don't have the diagnostic capability to take it on. Not sure what's the next step. I don't mind having a go at changing the cat, but it would be a shame if a) it didn't fix the CO, or b) an underlying problem ends up killing the new cat over time. Air filter is good, plugs and leads oil/filter are relatively new (did them when I changed the lash adjusters last year). What do I need to read the fault codes on this model? Your thoughts most welcome on this - many thanks in advance :-)