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Hints for fixing P0155 O2 sensor OBDII code on '95 NSX-T

22 September 2005
So if your CEL light comes on and its a P0155 code, a few searches will confirm that this is the Bank 2 (Front bank) Sensor 1 (upstream of the CAT in the exhaust manifold) 02 sensor. There are plenty of threads that discuss what brand/part number of O2 sensor to use and recommend spraying 'lots of PB Blaster', but I wasn't able to find any info in the searches I tried on how to remove the front upstream O2 sensor, even in the Wiki.

There are lots of sensor options however I just paid a visit to the Denso website and used their part finder to get the right part number and picked one up on EBay for about $35. The only difference appears to be that the OEM O2 sensor has a couple of rubber bullets along the length of the wiring harness that help it locate in two metal clips on the side of the cam cover.

The wiring connector for the front upstream 02 sensor is actually located on the front right corner of the front cam cover (as you are looking at the car from the rear). The first job when replacing the sensor with the approach I used below is to disconnect the old sensor at this connector.

Having already replaced a downstream sensor a couple of years ago, I thought this would be a straightforward job, however once you get the car up on jack stands, you have one of those 'how the hell is that coming out of there?' moments.

So here's what I did:

First, get the car up as high as you can on jack stands as space is tight under there. Take both rear wheels off.
Remove the small oil sump guard plate (two 12mm headed bolts and one 14mm headed bolt in the 'H' member)
Remove the exhaust down tube between the manifold and the cat (3 x 14mm nuts on the manifold end and 12mm nuts and a 14mm bolt head on the two "springs" on the cat).
I used a 36" breaker bar to loosen the three manifold nuts, so you will need some leverage as they have been heat treated over the years from all that hot exhaust gas. The nuts and bolts on the cat side didn't require too much leverage however I would recommend using hexagonal sockets if you have them to prevent rounding any of the nuts or bolt heads.
With the exhaust tube and sump shield removed, you can see the O2 sensor in the manifold quite easily and more importantly, you can get better access to the sensor with hands and wrenches etc. (not unrestricted, just better!)
The issue in removing this particular sensor with an O2 socket it that once the socket is on the sensor, there is insufficient room to get the breaker bar or ratchet onto the O2 socket due to the positioning of the lower cross member and the bulkhead.
So invest in a decent sized 22mm open end wrench (mine cost $6 at the local hardware store).
The curved shape of the small O2 sensor hole in the exhaust manifold heat shield makes it virtually impossible to get the circular end of the wrench onto the O2 sensor however you should be able to get the open end onto the O2 sensor nut.
Make sure you have disconnected the O2 sensor from its plug in the engine bay (to avoid any short circuits) and cut off the wiring harness from the end of the failed sensor. This makes it easier to remove when you free it up and spin it out of the manifold and stops you having to fight the 12" or so of wire between the end of the sensor and the connector in the engine bay.
In my case, once I had the wrench on the sensor, it came loose pretty easily (no heat, no PB Blaster).
If you need to use heat or PB Blaster, having the exhaust pipe off gives you very easy access to spraying PB Blaster into the threads of the O2 sensor from inside the exhaust pipe, and also allows you to aim very direct heat into the O2 sensor from inside the exhaust manifold. Had the sensor not moved so easily with the wrench, my next step would have been to use a propane torch (the kind you use for soldering copper plumbing pipes) to heat up the sensor from the inside for 10 minutes or so. You could also probably get a set of needle nosed vise grips onto the business end of the O2 sensor inside the exhaust manifold for extra leverage if all else fails.
The only other suggestion I would offer is to push the new wiring harness back up into the engine bay area (it helps if there is someone to grab the connector when you push it up into the gap between the engine and the bulkhead) before you spin the new O2 sensor into the manifold as it helps limit the wiring harness twisting up and trying to undo the sensor.
Snug the new sensor into the manifold with the open end of the 22mm wrench and then reassemble the exhaust pipe, sump shield and you're ready to clear the OBDII code. While the non OEM wiring harness for the new 02 sensor may not have the rubber bullets on it, you can still feel the clips on the lower edge of the cam covers and push the new harness loosely into those clips to keep it in place.
Even though this particular NSX has 163K miles on it, I didn't need to use new gaskets at either end of the exhaust pipe and once they were tightened back down, there were no leaks when the engine was running.