High CO readings generally indicate incomplete combustion. Contributing factors can be:
Faulty O2 sensor,
Faulty MAS,
Faulty PCV valve,
Clogged air filter,
Incorrect ignition timing,
Faulty fuel pressure,
Dirty/worn fuel injectors
In my own car a vectra i had to replace my egr valve due to emmission problems,just be very careful with avensis of that year *constantly check your oil prob every 600 miles*,there was a problem with valves etc with this model hence toyota extended there warranty on this engine.you didnt say what engine is in it petrol or diesel,there is an additive that you can add to your fuel tank (it slips my mind what it is now)to lower your emmissions on your car a reputable motor factor should have it.i can find out monday for you.just remember ...check oil every 600 miles they do burn quite a bit,im prob not very helpful here but if i knew your cars exact specs i could be more specific.dont hestitae to get back to me with more info ...regards