This code means that the VDO tachograph has detected a journey being made without a valid driver or workshop card in card slot 1, or with an invalid combination of cards in slots 1 and 2.
Code 28 is generated when the tachograph records vehicle movement but cannot associate it with a valid card. This is usually a card usage issue rather than a driver behaviour or technical fault.
Common causes include a driver starting a trip without inserting their driver card, inserting the wrong type of card, or a company or control card being left in card slot 1. An invalid combination of cards across both slots can also trigger this code.
Trips recorded while Code 28 is active may be logged as driving without a card, which can create compliance issues if not identified and addressed.
If Code 28 appears, check that a valid driver or workshop card is correctly inserted and that company or control cards are not in card slot 1.
If Code 28 continues to be displayed after the cards have been checked, the vehicle should be referred to an approved tachograph centre for investigation, as there may be an issue with the tachograph or card reader.
https://www.gov.uk/find-approved-tachograph-centre-atc
