At 126,000 miles with marginal fuel economy, I figured it was probably time to change my O2 sensors. The current O2 sensors appear to be stock; however the wire quantity and colors do not match the chart on Bosch's website for a 2001 A4 AWM. I checked the part number and everything, this is supposed to be the right one.

The new Bosch sensor has four wires (black, gray, and two white ones), that's the part that makes sense. The part that doesn't make sense is that my old O2 sensor has five wires (black, gray, white, yellow, and pink), these wires even have their own terminals in the wire harness that's mounted to the front firewall.

To make matters even more confusing, there are six wires that go into the wire harness from the ECU (or wherever else they come from), and one of them terminates in the harness. None of the colors of these six wires match the colors of the wires they join inside the harness.

Does anyone have any idea how to wire this thing? I tried calling Bosch, but they're closed until Tuesday. This thread is just for the pre-cat sensor/black wire harness. I'll do the post-cat one later, hopefully without as many problems as this one. Any help is appreciated.

 

Bump.

 

the post cat sensor doesnt affect fuel efficiency so you dont need to replace that one unless you get a CEL for 'CAT efficiency below threshold' or something like that.

the two white wires are the heater wires. the black is ground and the gray should be the signal wire. i think wideband O2 sensors should have 5 wires, are you sure you got the correct sensor?

 

I have CELs for heating for both sensor 1 and sensor 2.

I know what the new sensor wires are for, I just don't know which wires they are supposed to be connected with on my car. I'm using the O2 sensors that Bosch listed to fit my car. The part number on the boxes match the part numbers on their website.

What's a wideband O2 sensor? and why does it have 5 wires?

 

2000 and previous o2 sensors are narrowband- they have 2 heater wires, a ground and a signal wire. 2001 has a wideband o2 sensor which is more accurate. they have 2 heater wires, 2 wires for the lambda element and a signal wire. im sure the part numbers on the box match their site, but are you sure they are correct for your car? like this one
http://www.purems.com/products/product.php/II=1804

 

Every site I use to look up O2 sensors for my car comes up with the four wire sensor. So how do I know which wideband sensor to use?

I made the mistake of cutting the original wires, that's how I found out it didn't match up. Is it a bad idea to drive the car temporarily without the sensor connected?

 

I'm guessing the sensor in the link you gave me is a five wire sensor, seeing how it's only application for the A4 is from 2001. Great site by the way, I bookmarked it.

One more thing while we're on the topic. How is an air/fuel ratio gauge installed? Does it feed off the readings from the O2 sensors? if so, which one? or does it take readings from both?

 

you can not drive the car without the sensor.

do you have a haynes or bently manual to know which color wires do what?
i hope you didnt cut the wires above the connecter (the ones attached to the cars wiring harness)...... you should have cut the plug off the bad o2 sensor and spliced it into the new one. that way you dont cut up your wiring harness.

an a/f gauge just ties into the signal wire from the primary o2 sensor.

 

I have a Bently manual, and the wire colors it listed did not match the colors of the wires in my car.

By the connector, do you mean the wiring harness-looking thing that's attached to the firewall? If so, then no, I cut the original wires the same length as the new one attached to the new plug.

 

the wire colors listed in the manual are the colors of the wires inside the harness (if you lift up the sleeving on the ECU side of the connecter on the firewall), not the color of the wires on the sensor itself. it took me a bit to figure that out as well.