OK so MOT failed today due to CO emissions during natural idle but appeared to be fine during fast idle. Any help appreciated at this time need to get back on the road ASAP and cannot figure out what could cause this. Everything I have seen online shows CO fail at both natural and fast idle speeds.

Measurements

Engine Oil Temp --- Min 60 Deg C --- Actual Value 60 Deg C --- OK

Fast Idle Test
Engine Speed --- 2500-3000rpm --- Manual Check --- PASS
CO --- <=0.200% --- 0.101% --- PASS
HC --- <=0.200% --- 75ppm --- PASS
Lambda --- 0.970-1.030 --- 0,998 --- PASS

Natural Idle Test
Engine Speed --- 450-1500rpm --- Manual Check --- PASS
CO --- <=0.300% --- 0.807% --- FAIL

If it was my cat would I fail at both fast and natural?

I will admit my pre cat lambda sensor was broken for a good few month leading up to the test and I only replaced it 5 minutes before the test, could this be the cause and it needs "running in"?

Could burning oil cause this?

Could I possible fix this by removing my air filter for the retest so there is no restrictions, reducing possible leakage?

Any and all ideas welcome.