It is if they can see it:
"8.2.2.1. Exhaust emission control equipment
You only need to check components that are
visible and identifiable, such as diesel oxidation catalysts, diesel particulate filters,
exhaust gas recirculation valves and selective catalytic reduction valves.
If a diesel particulate filter has clearly been cut open and rewelded, you should reject it unless the vehicle presenter can show evidence that there was a valid reason to cut it open, such as for filter cleaning.
Defect | Category |
---|
(a) Emission control equipment fitted by the manufacturer missing, obviously modified or obviously defective | Major" |
So they can simply fail your MOT if they notice it is missing. not a prosecution.
A lot simply choose to ignore it but if you p!ss one off he could fail you.