Bunch of SES codes at once

[catsnet]

Hello AllI'm new to this side (Great site B.T.W. Lot's of info Thanks to all)The reason for the first message is.I have my 2001 QX4 3.5 for about 4 month now - no problem till recently.In the last 2 weeks I got a whole bundle of codes popping up almost every day now.First - the only things I’ve done to the car is Accel filter - right after I got it, and synthetic oil.Here are the codes I'm getting:

P0171 - every couple of daysP0321 - the latest addition, 2 days agoP1320 - couple of times in the last 2 weeks

As far as I see it, Air filter needs to be changed to the stock (Paper) one. The new spark plugs (Irridium) will be installed this weekend (160K KM is probably already overdue). New Fuel filter - just in case. I will clean My MAF at the same time and see if it works.I was also considering to get once of the distributors, but at the end don’t really want to have a “trial and error” stretched for a few weeks (Is their any ways to find out which cylinder misfires?).

SES always comes up on the Idling and once I'm on High 4X4. This is the only coincident I found, but once I read codes, their is 1 or 2 additional that I don't know the timing of. B.T.W the weather was up and down Temp wise. +5 to -30 C.

Please let me know what you guys think. What else I should check / consider

Thank you all in advance and sorry for the messy post

Alex