I've just joined the Forum and am also suffering with emission problems (with my Jazz, that is...) It's a 2005 1.2 litre engine, with 110K miles, which suffered a catalytic converter theft 3 years ago. A patented (non-Honda) one was fitted, together with a new exhaust system & O2 sensors and all has been well for several MOTs since. So far fo good....
At the last MOT it failed for headlamp beam (an easy fix) and emissions, which, to quote the tester, were "all over the place!" He did a diagnostic for free, which threw up a failed pre-cat O2 sensor error. He also suggested new plugs.
I went a bit over the top, and fitted eight new coil packs. eight new plugs and a new lama probe.... The test result was still a fail, with the emissions every bit as bad as before.
I'm a retired mechanic, so am not a complete novice with cars, but this has me foxed! Poor compression in one cylinder? Worn rings? Are there any other items in the engine that are worth examining and possibly changing? I love this little car, it's as cheap as chips to run (NORMALLY!) and this is the first serious problems it's had.
Any thoughts out there in Jazzland?