Nissan Micra Warning light - chain?

Nissan Micra k12 from 2004.
Have alot of trouble in the past 3/4 year. The yellow warning light are on all time and message said that I have serious engine trouble and have to go to a Nissan garage.
Every tread at the net say with one voice the chain has to be changed. The most expensive solution of them all £1100-1500. (If this is the only engine fault, why does Nissan not write that in the display)

No noise from the engine.
Different garages tried to reset the computer, but then the engine fail and stop unmotivated, also at highways....
After some stops the warning light starts again and i´ve been toldt, that the computer goes into a emergency program and then it can drive again.
Could it not be a faulty chain sensor? Why change a chain thats not to long and not make the known rattle noise?