I have a 99 Nissan Altima; I recently washed the engine because it was leaking oil from the gasket that covers the head by the spark plugs. I changed the gasket, the plugs; dry the entire engine including the distib. Cap, and now I have the engine light that reads
ENGINE SERVICE SOON. What might be the problem?

You might have left something disconnected when you worked on the car. Have a scan done to determine what codes are stored in the computer; post them here and we'll go from there.

I checked the conections, and found that the knock sensor was not doing a good connection, and that solved the issue
Thank you