Rite then
It is to do with the oil that attacks the metal content in the chain.
As the Oil gets older and used it makes a acid which in turn attacks part of the chain which in turns make the chain stretch.. We are only talking mm's here but it is enough to turn the engine light on. The cam and crank sensors detect where the engine is and if that is out of spec it will fault.
Also you may find you get a lumpy idle, this is the ECM trying to correct the problem. If you disconnect the crank sensor when the engine is running you will notice it will idle ok for a bit.
The chain can be changed with the engine in there and will take about 4-5 hours to do. ( I can them in 3 but there again I'm shit hot :thumbs: )
So £400 for the job is good and the rest of you with N16's change you oil on time and use the recommend Nissan Oil.