I have a number of problems.

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.