'03 QX4, O2 sensors, a thousand questions

[Mike W.]

'03 QX4 with about 130K on it now.

Recently on vacation the infamous CEL came on. Scanner said

Stored:
P0430 Catalyst System Efficiency Below Threshold (Bank 2)

Pending:
P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0430 Catalyst System Efficiency Below Threshold (Bank 2)
Calculated LOAD Value 77.25 %
Engine Coolant Temperature 188.6 °F
Short Term Fuel Trim - Bank 1 -4.69 %
Long Term Fuel Trim - Bank 1 0.00 %
Short Term Fuel Trim - Bank 2 0.78 %
Long Term Fuel Trim - Bank 2 -0.78 %

Which I'm hoping is just an O2 sensor. The code did go away a few days later and ~1K later or so has not reared it's ugly head again yet. But I know it's lurking and I'd just as soon get ahead of it. I've only had it a few thousand miles now but spoke with the PO and he didn't recall any being replaced. Bank 2, sensor 2 is a Denso, and didn't look all that good, pretty sooty. Bank 1 is a Bosch and looked much better. Pre cats appear to be Bosch, so I'm guessing the Denso is a replacement, the only one. I'm crossing my fingers it's a sensor problem, not a cat. MPG on the trip was just a hair over 18 which sounds about right for a mix of highway and 4WD roads.

So, I guess I need to replace all of them, even if the one oddball has been replaced before. And pre cat are 3 wire, post cat 4 wire. Seems odd, whatever. But how in the hell do you replace the pre cat ones? Only after making sure medical insurance is paid up and the local blood bank has my type in stock? Looks like my arms are going to look like I was trying to pet a feral cat, are there tricks to it or is it just keep taking stuff off until you can get to it? A long socket? I've usually just used a 7/8 wrench or a kind of crowsfoot speciality socket. I had a semi deep socket from HF one time but it was too short to get a headed O2 sensor out.

[Blindaviator]

They make a special socket specifically for O2 sensors. It's a deep socket with a slit cut out of one side to clear the wire. Dunno if that will make it easier or not as I haven't had to replace them on this vehicle.

[RR425]

I replaced the post cat ones and cats, never the pre cat ones. Yeah I bet they're a pain to reach. Probably need to remove heat shields from cats or the cats themselves. Should be able to reach them with a long wrench at that point.

[Blindaviator]

Not sure if this will help but I have a picture I had taken of the engine I swapped into my Pathfinder just before I disassembled it for upgrades. The water transfer tube is off but it does show the pre-cat O2 sensors:

[Blindaviator]

When I did the engine swap in mine I found it a lot easier to remove the drivers side exhaust manifold to pull and reinstall the engine. Pulling the manifold is really pretty easy.

1) Pull the 3 bolts off the exhaust post cat.
2) Remove the air box and air line all the way to to the intake.
3) Remove the heat shield from manifold.
4) loosen and remove all the nuts holding the manifold on.
5) Then use a properly sized torx socket to remove the manifold studs from the head.
6) Pull the manifold out with cat attached.

The passenger side manifold I left on the engine before pulling and for reinstall. Dunno how tough it would be to pull.

[Mike W.]

I can see the front O2 sensors. I know where they're at. Sort of. I might be able to get in there without needing a blood transfusion, but turning them to remove them is a different story.

I can see why there's not much in the archives, nobody want's to change them!

[Blindaviator]

I can see the front O2 sensors. I know where they're at. Sort of. I might be able to get in there without needing a blood transfusion, but turning them to remove them is a different story.

I can see why there's not much in the archives, nobody want's to change them!

Well the drivers side isn't too difficult really. If you remove the air box and air hose it will give you a lot of room to work with. When I dropped my engine back in a few months ago I had that O2 sensor out when I put it back together. I put it back in after the manifold was reinstalled and I was able to do it with a regular wrench.

The passenger side one doesn't look too friendly tho. You will likely need the O2 socket with possibly a long extension and a U-Joint wobbler to get at it. I haven't looked at mine too closely to see if there is enough room between the top of the O2 and the firewall to fit the socket and U-Joint or not. Otherwise it might be a long wrench from under to get it loose.

[rgk]

Would there be any reason not to pull all the studs out of both heads, move the exhaust manifolds aside, and pull the engine with one or both manifolds still in the engine bay, attached to the cats?

I can't remove the pre-cat heat shields to get at the bolts that attach the cat to the manifold.

If I can remove the driver mani with the cat attached, and remove the studs from the passenger mani, maybe I can remove the engine with the pass mani still in the bay.

[Mike W.]

I was able to change both sensors with the engine in place and without removing too much. Drivers side was not fun and passenger side was a full PITA, but it was doable with out removing the manifold or anything like that. The special socket I had to beat on since there is a little sheet metal flange on the factory sensor prohibiting it from sliding all the way on. Multiple extensions and a u joint. I think I might have been using short extensions in place of a longer one as kind of a wobble drive. I ended up bending the O2 sensor in the process, but I got it out. The new one wasn't easy to get in, but not as difficult as removal of the old one.

I think I used the kind of crowfoot type of socket on the drivers side and the slotted one on the passenger side.

[rgk]

OK, I got the driver's side manifold out! I needed this win.

Blindaviator, thank you. If it weren't for what you wrote, I would have gone down the wrong path.

I carefully removed the remaining studs from the driver's side head, removed the three fasteners at the downstream end of the pre-cat, and pulled the manifold-catalyst assembly out through the engine bay. I only had to unscrew the dipstick tube from the block and move it over. Everything else (air cleaner, harness, etc.) I already removed prior to starting on the exhaust.

In removing the remaining studs, I had to screw in an old stud with nut attached so that the manifold would remain flush against the head, and not slip down and put pressure on the remaining studs as I removed them. A 1/4" socket fit over the torx end of the stud perfectly (the socket end, not the drive end). I used a small (1/4" drive) socket driver to not apply too much torque at once. Once I started feeling some resistance from the stud, I applied lubricant, tightened the bolt back into the head, and repeated the process. It took probably ten of these iterations before I felt the stud was loose enough to remove it with comfort.

Next I will try to remove the remaining passenger side studs and slide the mani over as far as I can before removing the engine. I'm pretty sure the manifold won't come out on that side prior to engine removal.

Mike W., I plan on removing the O2 sensor after the engine is out of the bay.

Happy Easter everyone.

[rgk]

I got the passenger side manifold and pre-cat out! I'm just about ready to pull the engine.