by dshadle1, July 29, 2009
I am trying to replace the pre-cat oxygen sensor on my 2001. I ratcheted off the two 12 mm nuts that hold down the flange, but when I tried to pull the sensor/flange assembly off, the whole thing is being obstructed by something (something else coming off the exhaust manifold?). I can't quite see what it is. Is there a trick to getting this thing off? The flange jiggles, but it won't pop out. Do I need to remove any other parts?
Thanks in advance for any tips or advice.
Hello and Welcome the forums.
Should just be the two nuts holding the flange on the exhaust down pipe. There is a gasket in between, but if it is jiggling - then it means it is free from the downpipe. Could be a heavy buildup of carbon on the element itself that is holding the sensor in there. I'm also assuming that nothing is directly pushing against the wiring or the sensor body itself. Try looking at it from above and below - could be that there is a lock washer on there that is holding it in place.
Hello and Welcome the forums.
Should just be the two nuts holding the flange on the exhaust down pipe. There is a gasket in between, but if it is jiggling - then it means it is free from the downpipe. Could be a heavy buildup of carbon on the element itself that is holding the sensor in there. I'm also assuming that nothing is directly pushing against the wiring or the sensor body itself. Try looking at it from above and below - could be that there is a lock washer on there that is holding it in place.
Thank you very much for your reply. It was definitely free from the downpipe (re-attached it this morning). The obstruction appears to be above the sensor body and to the left.
http://i838.photobuc...81/100_1769.jpg
I've attached a photo if that helps. The threaded pipe-looking object appears to be in the way, and I can't seem to slide the plate under it. I admit, I wasn't trying very forcefully because I don't have a replacement sensor.
Thanks again for your help!
OK, good pic, I see it now. Those springs are part of the bolts that hold the downpipe to the exhaustmanifold. Looks like in your case, they have swiveled into the way of the sensor flange. Depending on ho much they shroud the flange, you might be able to shift the downpipe around to get it to clear the spring/bolt. Another option is to just remove those bolts and drop the downpipe to give you plenty of room and change the donut gasket while your in there. That donut gasket between the downpipe and exhaust manifold is a common problem area on our generation of Corollas. Tends to disintegrate over time, causing a leak right before the upstream O2 sensor - causing the sensor to think it is running leaner than normal.
OK, good pic, I see it now. Those springs are part of the bolts that hold the downpipe to the exhaustmanifold. Looks like in your case, they have swiveled into the way of the sensor flange. Depending on ho much they shroud the flange, you might be able to shift the downpipe around to get it to clear the spring/bolt. Another option is to just remove those bolts and drop the downpipe to give you plenty of room and change the donut gasket while your in there. That donut gasket between the downpipe and exhaust manifold is a common problem area on our generation of Corollas. Tends to disintegrate over time, causing a leak right before the upstream O2 sensor - causing the sensor to think it is running leaner than normal.
Great ideas, thanks! I might as well change that gasket, because I have had an intermittent P0171 (lean trim) that clears itself from time to time--hence trying to get at the oxygen sensor in the first place. Maybe this will help everything.
I'm assuming I can remove those bolts, drop the pipe, and change the gasket from above instead of reaching up from underneath? I started to get at the exhaust manifold by removing those rusty bolts from the metal shield (you can see one at the very top of the pic). My ratchet/socket couldn't clear the last one on the passenger's side because of the heat shield behind it, and of course the one combo wrench I don't have is 12 mm (thank you Lowe's). Is this going to be an easy fix of just unbolt, move around, and rebolt? Is it possible to position the problem bolt/spring into the correct position?
Thanks again for taking the time to work through this. Off to find a 12 mm combo...!
Yeah, you can get t it from above. I had to replace that same gasket some years ago - you have to be a contortionist or have smaller hands/arms and/or have a ready supply of extensions, and swivel joints + cobo wrenches to get it off.
Try and see if you can shove the downpipe around and see if it will clear the flange. That joint can take quite a bit of movement, so no worries about going He-man on it.
For the P0171 - I'd try cleaning the MAF sensor in your airbox - a pretty common occurance on lots of MAF sensored vehicles. Not something that is mentioned in the manual, as it is not considered a user maintenance item. But a large number of people have reported eliminating that CEL just with a good cleaning of the MAF sensor. There are several posts on this forum and well as general info on the net.
Good Luck
Thanks, fish, for all your help. I've been on vacation and couldn't get to this until recently. I took off the upper insulator from the exhaust manifold so I could see what I was doing and that spring+bolt just would not come off, even after a day and a half of PB Blaster (oh the smoke and smell when I had to take her out for a spin when all this was said and done...). Why did God invent rust? Anyway, I had a relatively short 14 mm box end wrench down there and wished I could have used a 2 foot cheater bar. Talk about cramped. I also tried shoving the downpipe around but it's on there pretty solid. I need to look at this from below and try to shake the exhaust pipe around down there. We'll see if I need to end up cutting this thing out--hopefully not!
The only other thing I can possibly think of is that the bolt has a concave head, not flat. Surely there isn't something I'm missing with that information? I don't see any kind of a nut on the exhaust explosion diagram.
All this cursing in the engine compartment for an oxygen sensor...oh well.
Thanks again, and I'll try to update.
DS
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