My MoT guys gently let off the throttle after the fast idle test to allow everything to stabilise gently. They take it from ~2500-3000 rpm back to idle over about 5-10 seconds rather than just taking the foot straight off. That allows the closed-loop control to sort everything out by the time the machine takes its reading. If you just close the throttle and hit the test button the mixture can still be a bit rich and cause this sort of problem, on older cats.
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