206 1.1 that won't pass its emissions test?!

[burnmw]

Not a 306, I know, but it's my little brother's 206, 1.1. Initially when it failed its MOT the list was both rear tyres, beam, headlight adjustment, exhaust leak and lambda reading was too high. Simple list, right? Wrong.

Changed the beam, put two new tyres on, adjusted the headlight, fixed the exhaust leak (gasket had gone) and thought I'd fixed the lambda reading.

So far we've changed, the lambda (for an NGK one), fitted a type approved cat, serviced it (oil and filter and plugs), changed the throttle body housing. I've tried searching for air leaks by spraying brake cleaner over the air pipes and no change to the engine speed.

First MOT results:

% Vol CO : 1.413 (Should be below 0.3 %) FAIL
ppm HC : 168 (Should be under 200pm) PASS
Lambda : 0.977 (Should be between 0.970-1.030) PASS

Other readings:

Air-fuel ratio : 14.350
% Vol CO2 : 13.86
% Vol O2 : 0.47


Last MOT taken:

% Vol CO : -0.005 - PASS
ppm HC : 13 - PASS
Lambda : 1.062 - FAIL

Other readings:

Air-fuel ratio : 15.606
% Vol CO2 : 14.68
% Vol O2 : 0.87

So, as you can see the CO volume has dropped dramatically after changing all of the above, but now the lambda reading is too high, how does that work?!

More importantly, anybody got any ideas what I could try next? I'm tempted to buy a cheap ODBII reader and see what the sensors are telling me.