Quattroism45

I've been getting conflicting answers about which set of sensors, precat or postcat, affect fuel trim. Heres the problem. Recently got these codes.

16795...Sec.Air Inj.Sys. Incorrect Flow Detected
16538...O2 Sensor Circ.,Bank2-Sensor1 No Activity Detected
17605..O2 Sensor Heater Circ.,Bank2-Sensor1 Electrical Malfunction

About 8 months ago I got these codes.
17605... O2 Sensor Heater Circ.,Bank2-Sensor1 Electrical Malfunction
17537... Long Term Fuel Trim mult.,Bank2 System too Rich
17535... Long Term Fuel Trim mult.,Bank1 System too Rich
16795... Sec.Air Inj.Sys. Incorrect Flow Detected

On other forums some say it's the postcat sensors. But from what I've learned it's the precats in conjunction with the maf that affects fuel trim, while the postcats just monitor the catalysts' performance. Right or wrong? I really don't want to change all four if it's not necessary.

So anyways, the codes seem to lead to faulty precats. My other question is how do I get to them? Thanks in advance.

Millatime

The Precat O2's are kind of a pain to get at. When I did mine i pulled the intake plumbing from the airbox to the throttle body and the coolant resovior.
Once you get all that off you should be able to see the exhaust pipes as they run back toward the firewall.

it was a pain to get at either of them from the top because of the angle they sit at relative to the firewall.

I got the car on ramps and did them from underneath. i had a crows foot style O2 Sensor socket and about 18" of extentions so i could clear everything. You could probably get them from up top with the right tools but I just didnt have them.

i started the car up and let it run for a minute or two to heat up the pipe to make it easier to break free. took some fussing with and some burns on my knuckles but once you break them free from underneath you should be able to reach in from the top and get them all the way out.

To install i threaded them in by hand from the top and snugged them up from underneath. Once i started the car they leaked like crazy so i let the car run for a few minutes to heat the pipe up and then snugged them up again and I was good to go.