i drive a mixture of vehicles and the other day had a strange issue when i got to site the tacho was saying invalid driver card entered but the slot was shut ive managed to get the card out and the chip was noticeble warm is it time to get a new card
No, it’s a gift. You can now keep ‘er lit 24-7 without fear of reprisal 
Assuming the card is still in date, (never overlook the obvious) and undamaged, try cleaning contacts by rubbing gently on clothing. Open tacho drawer and blow out any dust.
Check your card in another vehicle/check another driver’s card in that vehicle to see where problem is.
Failing all that report it, and run on a written record on tacho roll. Off the top of my head you need to do a printout at start, and end, of shift to show mileages and times. (So not exactly a free ride day!)
That’s a quick answer anyhow.
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lol even better the tacho unit has actually fried its self gone to do a company download as vosa wanted some data off it and the things died got a certificate of pelican daf saying its a broken tacho and has been condemed cost my boss 500 quid for a new one