hi

i have a number of problems that require assistance:
1. when i disconnect the oxygen sensor located at the exhaust manifold,check engine light doesnt come on ,nor does it show any error codes with a diagnosis machine.

2. i noticed that the spark plugs are turning brown on 2 diffrent occasions when i serviced the car, at the beginning of the white part,is this an indication of compression problems ?what can i do abt it?

3. i notice high fuel consumption 4km/l i tried cleaning the throttle body ,it worked however the problem recurred when my exhaust was fixed.
i have tried changing the oxygen sensor to no avail .
4. the fuel light comes on even when fuel is above "e" level then goes off as i step on the brakes only when i add significant fuel does it go off.