Nissan Repair: 2002 maxima Check engine light, faulty readings, o2 sensors

Question
I recently went to have someone change the spark plugs because I was getting bad gas mileage and power.  As we opened up the intake mainifold(lot's of work) we found out that I needed to replace the whole rear valve cover because it was leaking oil into the spark plugs.  Since Nissan does not make a seperate spark plug gasket, you have to buy the whole valve cover.  Since then I found out there were three seperate recalls on the car, which were also causing a loss of power amongst many other problems.  So now I have the check engine light back, they car is a little short on power and getting bad gas mileage.  So I took it to AutoZone and they said I have an EVAP error and cylinder 2 misfire(The Passenger-side rear cylinder, which is the hardest to get to).  So now that I already spent $600 getting the plugs done, do I need to have it all taken apart again?  Could it be the col or the injectore?  I did however use injector cleaner.  Someone told me that both of these things could be caused by the o2 sensor, is that true?  Well thank you in advance for any input.

Oan

Answer
Hi Oan -

This is true -
some injector cleaners leave residue and cause the O2 to give faulty readings, thus activating the check engine light. Please ensure that the cleaner used was compatible with O2 sensors.

Also, tell me, does the car run rough as if it is missing? Or does it just lack power like before? Please especially note when going uphill or accelerating. And, when you removed the plugs the first time, how did the electrodes look? Black, or brown, or white?

Please get back to me -
aloha
calvin