I'm not saying you won't find someone who will pass it without the cat. Its just that if VOSA demanded a re-test after it passed (within minutes), the garage (not tester) would lose their testing licence. Hence why they aren't willing to risk it generally. The testers and garage have no idea VOSA are following a test live. On the whole, not a bad thing.
When VOSA do spot checks, they don't inform the garage. They connect to the MoT mainframe wirelessly from a location nearby and watch the MoT as its logged (its all done on computers now). If they suspect anything (or even if the don't), they will attend the garage while the car is still in the station and demand a full retest whilst they watch. If anything was missed the first time, the garage is fcuked.
It must have been a faulty machine cause its not possible and car would pass with a cat gone,the co is messured from the cat along with other things.
same as if u have a leak in ur exhaust it will drive the lambda reading up!