Just recently had an M.O.T and it failed on emissions with a reading off 2.7co on normal idle and 0.9co at fast. The car is an STI TYPE RA with no cats. would putting two cats on make enough difference to bring it down? If not what else would I need to do? The LAMBDA senser passed OK.

 

Cats will help, but if its running the correct Lambda1 at idle it should only be 0.67 to 0.75% CO anyway, so its running rich, but it won't pass with cats anyway, so you'll need to fix both issues!

Simon