They have rudimentary computers in them these days - since unleaded petrol and the need to control the fuel/air mixture very precisely. They are called Engine Management Units or Engine Control Units - they connect to several sensors and if something goes above a set limit a warning light comes on - "Get The Car Serviced" it says.
The trouble is you never know if the spanner warning is a real problem or a glitch in the software. A garage will be able to interpret the data and advise on appropriate action - just turning off the light may be expensive.
I know nothing about cars...
Peccavi
February 2010