My engine light had came in Dec 2017, but i didnt pay any attention to it cause the car running fine. I thought it was the oil it needed an oil change but that wasnt the case. Heading to a friends house on Dec 22 2017 the car started to act up. It wouldnt pass 20 mph and the rpm would rev very high. So, i went to Auto part to have the code read and it came up with P0420 (Catalyst System Efficiency Below Threshold Bank 1) and P1800 (Variable Intake Air System Control Solenoid Valve Circuit). Dec 23 2017 I went to O'Reilys cleared the codes and purchased the Intake Air System Control Solenoid Valve Circuit. I unplugged the batter terminals and installed the Intake Air System Control Solenoid Valve Circuit. Reconnected the battery terminals started the car and let run for about 5-10 minutes. Then I went for a test drive and still the same problem(car jerking, rpm reving high, cant pass 20 mph). Went back to O'Reilys took off the new part, returned it and installed the old Intake Air System Control Solenoid Valve Circuit. I checked to see if the check engine light would come on after letting the car run for a bout 45 mins, but no check engine light. I went to O'Reilys again and they informed me that that the code P0420 could be multiple things such as the o2 sensor, MAF, Catalyst Converter and I dont remember the rest. I changed the up stream o2 sensor and had disconnected the battery overnight. The next morning I connected the battery, started the car, run it 10-15 minutes and then went for a test drive. Still the same problem no engine light SMDH !!!!!!!. Now the car has been sitting for about week now with out the battery being connected. Test drove and guess what same thing. Today is Jan 2 2018 10:58 pm and im fed up. Ive read many forms and no help.. PLEASE HELP SOMEONE PLEASE !!!!!

Hey there, welcome to NICO Club!

I think we can help a bit. I know we'll try!

Okay, you need to get the Factory Service Manual (FSM). We have it here and you can download it for free. So we got your back with that. This link is to each chapter:
http://www.nicoclub.com/FSM/Maxima/2007_Maxima/
go to each one and use the arrow symbol that appears in the upper right corner of the PDF to download it to your device. The FWD chapter is the Foreward and will have a table of contents. The EC chapter will have most of your engine computer issues explained, including by code. So, those are your starting points.

Scan the car for codes again maybe, or just go with what you already have. So far I see P0420 and P1800, is that all? For most if not all of the rest of your work, keep the battery connected unless directed to unplug it by the FSM. That will keep your ECU up and running and will help you know when you've fixed the car.

Each code will have an explanation of what the system is supposed to do and what possible causes are for it not working. Now, it does get into diagnoses instruction using consult tools that most don't have, but after that instruction, there is typically another set of instruction for diagnostics without a consult tool (like using a voltmeter). In addition to checking components, you'll be checking wires for continuity, voltage and good grounds.

Also, keep in mind that new cars have 'relearn procedures for throttle, idle and such. When you work on the intake or throttle stuff, you're supposed to 'relearn' the ECU. Those instructions are found around page 79 of the EC chapter.

Please let us know what you try and what you're up to. Good luck!

Note the possible causes, think about your car, the work you've had done to it in the past, your driving style and road conditions that led to the car not working well. That can help you narrow down where you want to look. Also, consider diagnosing the easy stuff before the hard stuff, obviously hoping that it's something simple!

You'll notice that O2 and MAF are not in the list... they're just guessing.