[W126 Coupe] check engine light is on

[Gary]

any thoughts   1991 560 sec 140k miles
 
in the past 2 months i've done some work i.e.
 
new fuel pumps
fuel accumulator
fuel filter
fuel pressure regulator
fuel injectors with seals
spark plug wires beru
spark plugs ngk
cap and rotor
 
can't imagine any of these setting off the engine light.  someone told me to check the O2 sensor but i don't know how.  don't know how to use a meter.  the thing is i replaced the sensor about 1 year ago along with the fuel distributor.
 
could the sensor go out?
 
what else could it be?  
 
thanks for the help
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