woland5

Hi,I have Accord 2003 v6. Recently if the car stays overnight after I start it feels like motor has trouble, it makes some sort of puffing noise, but after some time passes car warms up and it sounds great again. Maybe not all cylinders are working, I don't know. After I put it into drive I get a check engine light "p2240" thats O2 sensor, Sensor 1 bank 2. The car then drives fine and starts right up the rest of the day and on the second start check engine light goes away. Same thing every morning. There is also some trouble with switching gears and rpms are higher than usual after the first start, but again, after the second start that day is fine (as long as the engine is still warm). What I started doing is warming up the car first, then turn it off and start it up again. That seems to be working and no check engine appears.
I'd like to find out what is going on before I change the o2 sensor, my concern is that it might not be the sensor and instead something else that triggers the check engine to show o2 sensor code, when in reality it is something else (but I don't know, maybe these are exact symptoms of O2 sensor failure, maybe it fails gradually).
Also, in this situation, should I get new spark plugs? I read that if spark plugs are bad, I should get a check engine code telling to change them, but I am not sure.