I just put a new engine in my civic(I called honda to make sure it was the right one) and the check engine light is coming on with a code #1 which is oxygen sensor problem so I replaced it but it is still on and when the car idols it goes high to low rpms.

Sometime the light stay on after you fix a problem ,if the sensor was the problem the light should go away after you drive the car for several miles.

Sometime the light stay on after you fix a problem ,if the sensor was the problem the light should go away after you drive the car for several miles.
No if you have a CEL (check engine light) it will not go away you have to reset the ecu all you have to do if unhook the battery for about 5-10 minutes then hook it back the ecu will be reset. What model is your car ?is it a dx,ex,lx,cx,vx,si ?and what motor did you put in it ? The dx,ex,lx and si all have 4 wire 02 sensors the cx has a 1 wire and the vx has a 5 wire.that will make you have a CEL. Did you use the stock wiring harness ?