Mk5 fiesta 1.25 failed MOT on fast idle emissions, CO = 6%, lambda =0.810, HC = 230ppm. Tried changing the air filter, coolant temp sensor, first lambda sensor and fuel rail and its still no different.

Enigne runs fine and economy is still good although smeels really rich when cold, garage thinks it wants a new cat and post-cat lambda sensor, going to try the sensor but dont think the cat is the problem as its well rich anyway.

Anything else I could try changing? plugs and leads looked recent and all the plugs look good. Doesn't seem to have a MAF so struggling off other things to check??