Greg PAW - MS

ok, I read the 12v.org documentation on testing the o2 sensors and am a little confused. It mentions the 2 white heating wires and the black signal wire. The A4 I'm working on also has a 4th gray wire, what the heck is that one then?

The car is still generating the 3 o2 related codes after I tightened the sensor up slightly. I say slightly because it's only a fraction tighter than finger tight as it's simply not possible to use any kind of tool on the o2 sensor as it's surrounded by all sides by the engine, hoses, tubes, firewall, etc. I can get a wrench or ratchet on it but then there is literally less than 1degree of rotation possible before you hit an obstruction no matter how you go at it.

I'm going to have to have them bring the car in and remove the entire downpipe from the car to fasten up the sensor. Hopefully this is the problem, I don't know how to go about testing the sensor not knowing what the heck the 4th gray wire is.

P.S. I now _HATE_ o2 sensors, ktnx

Jretal

That should get you enough space to work...

That's what we had to do on my brother's '95 90 when we replaced his O2 sensors...

Or you could get an O2 wrench (has a slot in it for the wires) and a longer extension... maybe throw in a universal joint?

eep

After all of my huffing and puffing on the instructions there about using 3-wire sensors, the post-cat sensors are 4-wire. The 4th wire, as I should have remembered earlier but didn't (sorry Greg) is a signal ground for the ECU to use instead of the manifold (chassis) ... used on after-cat sensors because there's too much resistance between the sensor and the chassis.


I've got a couple of notes on the instructions about the post-cat sensor but nothing really informative. That's something I've been meaning to fix but I haven't done it because I'd like input from someone with an A4 about how exactly things fit, what's where, etc.


Any takers for that here on the list? Please e-mail me (web@12v.org) so I can fix that finally.

Greg PAW - MS

the problem is that there is only about 2-3 inches of space in any direction around the sensor. I have an o2 sensor socket but once I put it on there I'd need a ratchet with a handle no more than 1" long in order to turn it!

Greg PAW - MS

they were working fine, removed them, put them back in, and BAM, 3 codes... I know the post-cat sensor isn't as tight as it should be, is that really the likely cause of the codes? It's not like it's hanging loose or anything...

I take it I can still test the o2 sensor by checking the voltage modulation between the gray and black wires (instead of black and ground since the gray _is_ ground)? Then again if it's not in tight enough then it won't send the right signal, right? And why would tightness have such a huge effect on these things???

Thanks a million. This is just getting very aggravating as I've almost spent more time trying to fix this freakin' O2 sensor than I have to remove and replace the entire transmission. I really feel like I'm letting this customer down.

eep

It's weird; the collection of codes you gave suggests that the thing just isn't plugged in. Maybe a wire got crimped or melted or something? Since the signal gets its own ground wire, it's less likely to throw a code just for being loose. However if it's not tight enough it might work its way out. Can't help you on that one ... one look at the cramped hood on the B5 cars is enough to keep me away from them

Here's an electrical check for the 4-wire sensors used in the 12v. The 1.8t uses a different type of sensor than the 12v (heated planar type/LSF in the 1.8t vs heated thimble type/LSH in the 12v) but the checks might work OK; it should at least give you an idea. Hopefully the pins are the same, anyway:




Disconnect the harness plug for the sensor. Looking at the plug, with the rounded end on your right, the pins go 4-3-2-1.

(ignition off) There should be ~0 ohms resistance between pins 3 and 4 of the sensor's connector. (3 and 4 are signal ground and signal, respectively)

(ignition on) The voltage between pins 3 and 4 of the *harness connector* (on the car, not the sensor) should be between 350mV and 450mV (goal = 400mV).

(ignition on) The voltage between pins 1 and 2 of the harness connector (on the car again) should be 12-14 volts (these pins are +V and GND for the heater)

(ignition off) The resistance between pins 1 and 2 of the sensor's connector should be 2-5 ohms for a sensor at ROOM TEMPERATURE. The resistance shoots way up as the sensor heats up so no promises on that one.

Jretal

Sorry, not trying to frustrate you further. I know if I've been staring at something too long I start to run out of ideas... so I'm throwing some in your direction.

Out of curiousity, how did you get it out?

Also, the crow's foot will enable you to free up more space I believe than the O2 socket would... might be something you wanna look into.

Greg PAW - MS

with the sensor in it. keep in mind that this is a 2000 1.8t. the sensor is located behind the engine towards the passenger side pointing towards the driver side strut mount with the tip of it about 1" from the firewall and a whole bunch of tubing and piping on the other side. it's also somewhat underneath the firewall so it can't be accessed from above either (not that i would help as there's only a 1-2" gap between the firewall and the back of the engine.

in short, it sucks if I were to build a B5 just for full out racing use, I wouldn't use the 1.8T just because it's impossible to do any kind of work under the hood with ease.

Greg PAW - MS

muchos gracias!

eep

... don't let anyone talk you in to taking me seriously


Did any of that stuff help? I think that the sensor being loose and the error codes aren't related, except that if the sensor is bad you don't want to go through the trouble of tightening it and then have to take it back out