Carpoof

I've got just over 210K miles on my '97 4.2 and I think it's about time to replace the oxygen sensors. They are the originals as far as I can tell (bought the car with 150K on it). Haven't gotten a check engine light, but my MPG are down a bit. I looked at the engine and can't see the oxygen sensors clearly. Is it a pain to replace the front sensors? What all has to be removed in order to get to them? Any advice is appreciated. Thanks in advance.

silverd2

They are probably not the originals, unless you check engine light is burned out (possible on pre-facelift A8). Does the CEL come on with ignition switch, before you start the car?

Bentley says the exhaust downpipes have to come off to change the fronts (only ones worth changing since the rears do basically nothing...fronts adjust fuel mixture)...BUT this not so, just need a the proper extensions and a O2 sensor socket (cheap). The drivers side should be easy to see, if you know where to look...the passenger side is a little tougher.

Here is a rough description of replacement the easy way on a facelift A8 (similar) >>>

http://forums.quattroworld.com/a8/msgs/49929.phtml

P.S. Bosch universals (for your year car) are cheaper and exactly the same sensor...you just have to splice in your old electrical plug (easy and connection provided with kit).

that_tg

I have the full repair record from when my car was new. Last fall I saw a noticeable decrease in mileage. I thought the same thing you did. (190,000 miles) Original sensors. Time to change them.
It is an easy job to do like silverd2 said.
Sorry to say I did not get any better MPG's. Still feel ok that I changed them.
I found the sensors with original plugs for maybe 5 bucks more each. I know, most would say, why spend the 10 dollars? I had visions of getting the job done and one of the wires being loose... Maybe that does not happen??
Good Luck!

silverd2

It's odd (don't know why) but the sensors for prefacelift (32V) cars are priced completely different...32V universals are much higher $$ than 40V versions.

40V Bosch universals can be had for $50 (or less) each, when Bosch OEM plug-n-plays are double (or more) the price at the same seller.

that_tg

I remember doing a lot of searching for the right price. If you have trouble I can try to find the place I ordered from.

Carpoof

Thanks for the input guys. I looked a little closer at my sensors and I am 90% sure they are the originals. I got service records with the car when I bought it from the two previous owners and the O2 sensors had not been changed. My CEL does work (It's actually on for a P0441 incorrect purge flow that I haven't figured out yet, it's not the N80). I found some Denso brand sensors that are exact fit for $30 each so I would probably go that route. I'm starting to lean towards not changing them since it looks to be a PITA, I can see the wires going to the sensors, but I can't imagine being able to get a wrench in there to change them. Alldata calls for 1.3 hours per sensor to change. I just ordered some new Bosch OEM style spark plugs so maybe that will be enough to help the MPG (Currently have NGK Iridium plugs with 40K on them). My mileage actually isn't too bad, but when I first got the car, I could manage 28-29 on the highway pretty easily, but now I'm down to about 25.

silverd2

If you decide to change the fronts (HIGHLY recommended...sensors that old can NOT be functioning properly...code or no code), I learned the method I used on the 40V from several others who did the same on their 32V motors (like yours).

Cheap tools with extensions will do it easily. Alldata, Bentley and Audi dealers are full of crap in this case...all 3 get exactly the same info straight from Audi. NO one changes those sensors by dropping the exhaust down pipes...not even the dealer, but they'll you charge as if they did...that's REALLY the way it works.
Put an Audi tech on a lie-detector and you'd know the truth:
Their records have to show that they went by the book (liability), but they take the shortest route for profit...and because it just makes more sense...minimum disassembly.

Anyway, my car seemed to running perfectly and getting good mileage. I only gained about 1 - 1.5 mpg (which ain't bad), but much more importantly WAY smoother idle and low rpm smooth power...like new again...hadn't noticed the slow decline in running quality till I did the job.

Suit yourself. The job is VERY doable from above. An 02 socket is only about $7 ...available all kinds of places.

Carpoof

I may end up changing them next month when I get back from vacation. I just didn't want to spend two hours to change a couple of sensors that should be easy to take out. I did put in a set of Bosch OEM spark plugs last night (I had NGK iridium's for the past 40K miles.) What a difference! The car has more low-end power, it shifts better, and I am getting 3 MPG better in town driving so far. I've noticed the highway mileage on the way to work was about the same so far, but the instant MPG's climb a lot faster during acceleration so my average is higher.