Repaired a three cylinder Corsa last week.
High CO, high lambda. On a hunch I blocked the tailpipe, found it blowing at the joint to the back box. Fixed that and then just got high CO, very careful listening at the front sounded like a small blow on the manifold when the throttle was snapped open. Took off exhaust pipe and found the seam had a hairline crack at the input to the cat, which a line of weld fixed. CO after repairing the exhaust was 0.04%, lambda 0.997. Thing is it really didn't sound like it a had a blow when it came in.
HTH Shaun