PDA

View Full Version : How do you diagnose which o2 sensor is bad?



Kyle H
05-03-2009, 07:34 PM
Like the title says, if your o2 sensor is bad (vagcom says so), how do you know which one to replace?

ghost6303
05-03-2009, 07:45 PM
primary is the one before the car, secondary is the one after the cat. vagcom will tell you which.

Kyle H
05-03-2009, 08:10 PM
Oh well it's not actually throwing an o2 sensor code, it says my o2 sensor failed the readiness 'test'.

Does that mean it's not actually bad?

Poopie
05-03-2009, 08:15 PM
did you recently clear the codes?

gabeguimaraes
05-03-2009, 08:18 PM
I have the same code. I'm tired of this emission, running rich, sensor bullcrap =/

tdn
05-03-2009, 08:22 PM
It's not a code, it's a readiness check.

If you have a CEL, it will specify which sensor.

If you recently cleared a CEL, you will fail certain readiness checks until you drive a certain amount of miles.

Of course, if you drive beyond that certain amount of miles (i forget how many atm) and you still fail, your car will throw a CEL.

gabeguimaraes
05-03-2009, 08:27 PM
Got you. I understand now but if I failed the readiness check for a sensor, does it mean I have to replace it or should find the cause and fix it before?

Kyle H
05-03-2009, 08:28 PM
I cleared the codes a good 1000 miles ago, no CEL, no engine codes, and it shows that it fails readiness. (the cat and the o2 sensors actually).

tdn
05-03-2009, 08:33 PM
@ gabeguimaraes
If you failed the readiness check and your car has driven the required distance... You have a CEL. The 'code' from that CEL should tell you which (and if it is in fact an) o2 sensor. If it's one or both of the sensors, replace them. If it's your cat, then have it replaced, or try the anti fouler fix (search)... well whatever it is, put the code into the search and learn.

@ Kyle H
That's strange, to me at least. Is that the only thing failing?

Kyle H
05-03-2009, 08:37 PM
The O2 sensors and cat are the only things failing readiness... no codes at all though.

http://i146.photobucket.com/albums/r261/snowmanhitman/ruhroh.jpg

tdn
05-03-2009, 08:39 PM
Oh ok, if your cat has failed also, I'm banking your cat has failed and it's not your o2 sensors (which would have thrown a CEL)

I'd wait for some others to chime in before you do anything, that should be a given.

gabeguimaraes
05-03-2009, 08:43 PM
This is the problem tho, I've searched like crazy and went through my Haynes manual also.
All different info I gathered: I'm running rich on bank 2, some causes are that I have failed MAF (only couple of months old), also high fuel pressure, failed O2 sensor or catalyst efficiency low bank 2 (driver's side only couple of months old).

Another thing, I think I might be leaking oil past my valve guide seals....

Kyle H
05-03-2009, 08:46 PM
This is the problem tho, I've searched like crazy and went through my Haynes manual also.
All different info I gathered: I'm running rich on bank 2, some causes are that I have failed MAF (only couple of months old), also high fuel pressure, failed O2 sensor or catalyst efficiency low bank 2 (driver's side only couple of months old).

Another thing, I think I might be leaking oil past my valve guide seals....

are you burning any oil? IIRC blue smoke out teh exhaust proves that. If your MAF is bad it could be reading high on the air coming into the engine, which would make you run rich, which could toast your cat from it having to try to burn up all that extra fuel.

gabeguimaraes
05-03-2009, 08:52 PM
No smoke. I'm leaking oil (hence the burning smell) valve guide seals and cam seals.
Can a MAF just go bad after only a couple of months??? I don't think that could be my problem.
My instincts tell me that this whole problem might be because of the valve guide seals letting oil leak..

walky_talky20
05-03-2009, 09:04 PM
I believe you would see a code if the cat was failing - "cat efficiency below threshold" or similar. How many miles are on the car?

Does anybody know about logging the data output from the rear 02 sensor? That might tell you what's going on.

gabeguimaraes
05-03-2009, 09:14 PM
Yup that's what it says.. I'm pissed at the fact that I spent $100 for a cat installed and now its gone again....
Best bet for me is wait till september when I'll have some money and just replace all leaky seals, o2 sensors, the cat and timing belt job...
Thanks

Kyle H
05-03-2009, 09:18 PM
I believe you would see a code if the cat was failing - "cat efficiency below threshold" or similar. How many miles are on the car?

Does anybody know about logging the data output from the rear 02 sensor? That might tell you what's going on.

Just a hair under 150k... I need to get a hold of a vagcom and do some logging.

gabeguimaraes
05-03-2009, 09:25 PM
btw...I have 131k