Error 20 relates to a motion sensor fault, which can affect how vehicle movement, speed and distance are recorded and will cause issues during roadside checks or compliance reviews.
To make sure this is a genuine fault, drivers should turn the vehicle ignition off and back on and see if the error clears. Additionally drivers can check the cables connected to the tachograph for loose or incorrect connections.
If the error remains, the vehicle should be checked at an approved tachograph centre:
https://www.gov.uk/find-approved-tachograph-centre-atc
Further details on the error can be found here:
https://www.dtco-user.com/errorcodes/2.0/index.php?lng=en&site=20-29