Dangerous MOT faults were introduced in May 2018. Issues which cause an immediate safety concern are recorded as dangerous and the vehicle should not be driven until these are repaired.
Examples of defects which are recorded as dangerous include excessively worn brake pads or tyres, a broken horn, or lights which aren't working.
You can check if a dangerous fault has been issued by using our Check MOT History service.
Dangerous faults are highlighted next to the relevant notice, under the "Test Details" section: