How do I turn off the engine light of my 1996 Discovery?

If the light keeps coming back on there is a code stored in the computer of your car. The code needs to be retrieved using a scan tool, the fault fixed and code then erased. After the codes have been erased the computer needs to see certain driving criteria met called drive cycles where it tests the various emission components in your car it then passes what are called "monitors" and then the car is ready to have a Smog Test done. It could take several weeks in some cases to get the car to pass the various "drive cycles".

Disconnect the battery to clear previously stored fault codes.