Avoiding unknown periods
The simplest way to avoid unknown periods is making sure the driver card is inserted any time the vehicle is being driven, and re-inserted promptly after any temporary removal. Keeping the card in for the full trip, including stops, avoids most of them altogether.
A few habits help with this:
Insert the card before starting the vehicle, not after.
Re-insert the card right away after a border check, ferry crossing, or any other moment it was taken out.
Check that the card is fully inserted and the tachograph recognizes it, rather than just placed in the reader.
Report a damaged or faulty card as soon as it's noticed, since a card that isn't read properly can create unknown periods even when it's inserted.
When they keep happening
If unknown periods keep showing up for the same driver, it's worth a closer look.
Recurring gaps are usually a sign of one of two things: a habit, like forgetting to reinsert the card after a break, or a technical issue with the card or the tachograph itself.
Comparing when the unknown periods happen can help narrow it down. If they show up around the same type of stop each time, it likely points to a habit that can be corrected with a quick reminder.
If they happen at random or the card seems to have trouble being read, it may be worth having the card or tachograph checked. Catching the pattern early makes it easier to correct before it adds up to bigger compliance issues.