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High Co Emissions On Fast Idle Test

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I've recently taken my 2001 Celica for an MOT test, it passed everything except the fast idle CO (limit is 300 mine was 400 & 550 on 1st & 2nd test), natural idle test passed fine.

The other values on fast idle are fine too, low hydrocarbons and lambda reading within correct limits.

I've tried cleaning the MAF sensor and I've confirmed using torque and a OBD2 reader that it's changing values with engine speed. I've also cleaned the air filter even though it appeared pretty clean (has a K&N filter). I took it back to be retested and it still failed on only the CO.

For the MOT 2 years ago it failed with high HC, so I changed the cat and both Lambda sensors. It passed that MOT and the year afterwards without any issues.

I've just tried putting some Wynns Clean burn emissions reducer in, however this has had an awful effect of giving it a rough idle and causing high negative fuel trim, so will need to flush all of that through before a retest. I'm still doutbful it'll pass the MOT though.

Can't find any exhaut leaks and all hoses/pipes appear to be in good order.

Does anyone know what is likely to be causing the high CO reading? Would it be worth removing the air filter for the retest?

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