The police regularly conduct roadside checks of heavy vehicles and can stop any vehicles found to be unsafe from going back on the road.
If your vehicle has a fault, the consequences will depend on the safety risk it represents. The guide categorises these as 'dangerous', 'serious' or 'moderate'.
Depending on the risk category, you may be:
instructed to repair your vehicle
instructed to park it up
issued with an out-of-service order and accompanying pink or green sticker.
Usually, if you get a sticker your vehicle will need a new certificate of fitness. However, the inspection officer may issue a green sticker and instruct you to repair the fault(s) within a set timeframe without the need for a new certificate. In this case you'll need to prove to the officer that you've repaired the fault(s).
See the full list of possible heavy vehicle defects and the consequences in Heavy motor vehicle categorisation of defects, a guide for roadside inspections to help ensure the detection of faults and the consequences applied are consistent.