Cause of high carbon monoxide emissions?

Clogged air filter?

If the lamdas 1 then it's more likely to be the cat, ithe air filter can't stop that working.

My first reaction was to change the plugs and filter, but they were only done about 2000 miles ago.

Cat isn't much older, but did receive a bit of a clout thanks to a gate post stub.

Can a knackered cat exhibit this behaviour? Would've expected high figures across the board, and not much difference between 1st and 2nd fast idles?

Just checked the emissions from 2 years ago. First fast idle was basically the same (failure) reading. 2nd was a pass at about 0.09%. Natural idle at that stage was 0.05%

Paul

Cat doesn't affect lamda, working or non-working it'll still be 1.0, non-working sensor will screw your lamda, non-working ignition or oil will srew your H?C. Cats don't normally die that quickly though, if you've really mashed it then it could be dead . They're about £90 + the Vat

• fitting. & you can get tradein on the old one.

have you checked that all the breather system is ok? particularly that little pipe that goes on low on the inlet manifold, they used to clog up a lot on 16v cavaliers, they even modified it so that it was heated to reduce the clogging, and of course a minor exhaust leak can give a high reading.

mrcheerful

A small exhaust leak upstream of the O2 sensor will suck in some air and fool the sensor into adjusting the closed loop mixture too rich and hence high CO. That gate post clout may have broken the manifold seal.

That seems plausible. The CO reading is too low for the cat to be totally knackered.