There are *many* causes, too many to list here.
There is no need to guess why the light is on, the car's computer will
output one or more trouble codes to a piece of diagnostic equipment,
which will help a technician diagnose the root cause of the problem.
 The computer also outputs a huge amount of data in real time, showing
the operating parameter of the engine control system, which further helps
a qualified technician to diagnose and fix whatever is wrong with the
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