Failed emissions test High CO (0.6) and Lambda (1.033).Fast idle. 0.23 CO at natural idle. Checked for switching of O2 sensor, appeared normal but exhaust lacking the characteristic cat'smell' Replaced Cat with aftermarket unit. Emissions eventually settled at 0.07 CO, 1.022 Lambda fast idle 0.01 CO at natural idle. Took for emissions test and failed again with similar results. I took the car for a fast run to local station whereupon it failed again despite an extended fast idle. Strange thing is though occasionally all levels drop to normal briefly before returning to high levels. Carried out some more tests all seems ok except there is vacuum flow through the Carbon canister valve all the time. Venting to atmosphere sends the O2 levels soaring, blanking off, causes engine to slow without much change to emissions.The tailpipe doesnt seem to get hot. I have been told that some cats require 'conditioning' with a hard road test before they will fire up properly? Sorry to ramble Andrew