02 Audi A6 4.2 o2 sensor replacement

[snopro711efi]

The check engine light came on and I checked the code and I have 2 o2 sensors that are bad. The codes are: P0135 Bank 1 sensor 1 P0155 Bank 2 sensor 1 Where are these sensors located on the motor? Can you access them from the top or is it easier from the bottom? Thanks for your suggestions.

[culture]

the 2 sensors are located in the catalytic down pipe. one upstream and one downstream. i haven't pulled mine so i don't know the easy way to get to them.

[djktno]

...it's right there on the exhaust, post-manifold, visible behind the driver's side cylinder head *if* you gently move the coolant reservoir out of the way. The upstream sensor on that side has a black connector with 4 wires about mid-bay on the firewall. The other three connectors (bank 1 upstream, bank 1 downstream, and bank 2 downstream) are all collected together on the firewall on the passenger side (black, brown, and green).

I got it loose with an offset oxygen sensor wrench (http://www.autozone.com/autozone/acc...requestid=9176) + 3/8" extension (12") through the tangled mess of hoses at the back of the engine. The top of the sensor is pointed up and toward the passenger side of the car which makes for an odd angle of attack.

Once it broke loose, I unscrewed the rest of the way with my hand jammed down between the firewall and the back of the engine. My hand took a beating back there - lots of sharp edges and the like, so be careful.

One thought though - is there a chance that something else (fuse, relay, etc) is going on? Seems odd that you'd get codes for both upstream sensors at the same time, unless your CEL has been on for a while and they've now both gone bad, or you're around the useful lifetime (around 100k miles, I've heard). Check these items out first and save yourself some hassle and bruising.

I found some decent information: http://www.vaglinks.com/Docs/Audi/Mi...esting_DIY.pdf
and here
http://www.vaglinks.com/Docs/Audi/B5...r_Checking.pdf

for background. The stuff for the B5 S4 is so close (for testing the O2 sensor readings) that you'll at least be in the ballpark of what readings you should see on your engine methinks. It at least gave a good frame of reference for me.

I'll be getting into the bank 1 side shortly, so I hope it goes smoothly.