If the blow is slight it won't double the CO, was the engine defo hot, ie fan had been on but gone off, a few degrees around the normal can make a big difference.
Any leak into the exhaust system is not going to help, in between every puff out there is a vacuum pulse drawing air in, lambda could be getting grumpy.
It may also just be the lambda, which my money would be on, have you means to check the switching speed, they tend to switch much slower when duff, and can give a decent gauge reading(voltage) but still be duff.
I assume the air filter is nice and clean/clear ?, mot place should have a good idea from full set of readouts.
If you need a new lambda, I can recommend ecolambda, pattern ones but do work, use these myself, around £25 work 100% perfect need to crimp some new ends on though, but are supplied.
tabetha