Around 2006, solder containing lead was banned for use in electronics, before then, most solder joints were 40% lead and 60% tin. Many companies such as smart decided to be environmentally friendly and move to lead free solder which consists of either 99% tin and 1% copper or 95% tin, 4% silver and 1% copper (average).
There are 2 big problems with lead free solder that can cause serious electrical problems such as transmission faults, CAN bus errors, dim or non working headlights, faulty central locking, horn failure, cabrio roof failure etc.
Out of the 2 problems, this is the least likely but well worth checking anyway.
Firstly it suffers from an effect called "tin whiskers". These hair-like structures grow from lead free solder
over many years, up to 1 cm long. If they touch any other electrical component it can cause a short circuit.
It was this reason why lead was added to solder in the first place. Adding lead stopped these tin whiskers
(researched by NASA) so even today, all NASA projects use lead solder because no one has yet worked out
how or why these tin whiskers occur. They are 200x thinner than a human hair.
This is a super-macro photo of the PCB inside a Roadster/fortwo key. There are plenty of tin whiskers forming.
This could be a reason as to why keys suddenly go out of sync with the car.
Unless you have a jewellers loupe and a lot of time, instead of manually searching the PCB for these fine hairs,
why not just go for the cluster bomb approach! Take a dry toothbrush and brush over the solder side of the
entire board. This will break the weak tin whiskers. Blow the board to clear any broken off debris. Make it a
dry blow, not the sort you use to blow out birthday candles when you don't want to share your cake.
You could use compressed air in a can or even electrical cleaning spray if you have such things.
This is now becoming a common problem, mainly in the SAM unit but has been seen in other electronics.
Lead is a soft metal, much softer than tin. So, the less lead you have, the more rigid the solder joint.
Rigidity isn't a good thing in items that get hot and cold or move around a lot. The SAM unit does both.