I have a 93 Maxima SE.. Mostly stock (except me in the Car)...

Till recently I was getting ~ 23-25 Mpg.. Now it is reduced to 19 Mpg..
I thought Oxygen sensor is faulty. Then I did the diagnosis... During the diagnosis I ran at > 2000 Rpm.. The CEL is ON(lean condition).. When I stop pressing the peddle it is OFF (rich condition). The engine was warm..

I don't know why it is not entering closed loop mode (blinking at more than 2000 Rpm).. I checked the DTC's. It showed 55.. so no DTCs..
Is it because of old (tired) Oxygen sensor
What else could be wrong..? I checked Coolant sensor.. It showed correct resistance.. I don't know about thermostat.. What will happen if it is struck (engine over heat.. or any other signs..)

I am confused.. No DTC but bad mileage.. Engine runs rough at idle.. (could it be PCV failure)...

I am counting on your answers...