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O2 sensor harness wiring - help

20 March 2023
- New owner
- 1991 NSX
- Failed emissions a few times over the last few weeks, trying to chase down why.
- No check engine, no stored codes

So I have been trying to track down why I am failing emissions and I went to take a look in the engine bay found the engine side harness leading to the rear 02 sensor MacGyvered together.

Would this be causing issues, even without a check engine light?

Also, there seems to be another set of wires leading off of the o2 harness to the elsewhere under the rear head, anyone know where those go?


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According to the 1991 service manual, on the rear sensor the heater lines are blk/yel and red/ blue. In your third photo it almost looks like the blk/yel wire is severed. If the heater circuit has a bad electrical connection the heater may be under performing. If the engine is running at high load exhaust heat will get the sensor into operating range; but, if you sit around idling before going into test the sensors may not be up to temperature. The ECU has a heater failure detection; but, a bad connection may fool the ECU into passing the heater check even though the heater is not really doing its job.

The Grn/wht wires from the sensors are the sensor element grounds. According to the wiring diagram they are shared with the throttle angle sensor and the ignition timing adjuster. I don't know exactly where they split off. The Blk/yel wires are the heater grounds and they are also shared with the EACV and speed sensor.

Which emission test are you failing? If its NO I would be inclined to first check the EGR valve. If the cats are original they might be past their best before date. Is the engine stock?
Thanks for the background.

Currently failing NO @ 15mph, Max is 549, I was at 668

The car has 61k on the clock, no modifications
If it is exceeding NO limits it might be your EGR. The EGR does not open until the engine hits a certain RPM / load level and 15 mph might be below the point at which the EGR opens - depending on what gear you are in. 15 mph seems low. I thought the test speed was around 25 mph. I have no direct experience with this so it would be useful if some other members could advise on the particular operating / test conditions where a bunged EGR valve causes failure.

The EGR valve has a lift potentiometer on it and if the valve is not opening when the ECU asks it to open it should trigger an error code. Even if you don't have a CEL have you checked for stored error codes in the ECU - if you reset the ECU these codes will get wiped out of memory. Even if the EGR valve is functioning and the lift pot is reading the correct values, on a 42 year old engine the EGR passages may be obstructed / blocked which prevents the EGR valve from doing its thing. There are no error codes to detect a blocked EGR passage. I think physical inspection of the passages might be the only option there - a unpleasant job based upon where the EGR and its plumbing is located. Somebody who has gone through this may be able to give you some guidance.

A failing catalytic converter will cause NO compliance issues. How are your CO and HC numbers? If they look good the catalytic converter might be OK and you NO compliance problem may be EGR related. Depending on your jurisdiction, NO compliance on older NSXs can be a challenge.

The AFR of the exhaust entering the catalytic converter has to swing above and below 14.7. NO clean up works best when the AFR is below 14.7 (running rich) and CO and HC work best when its above 14.7 lean. If for some reason you are running on the lean side all the time that can screw up the NO clean up; but, your HC and NO numbers might be nice. Buggered O2 sensors might contribute to that problem. Unfortunately your 1991 fuel trim values are not easily accessible. Being able to see those might give you some idea what is going on. Sticking a wideband up the tailpipe with an adapter might give you some clues as to whether things are off.

Unfortunately, no definitive answers. Just lots of places to look.
Thanks for the reply. So yea it passed the 25 mph and failed the 15. I passed on my CO and HC number so the cats seem to be operating as expected. I am actually in the garage now trying to pull the EGR, the bolts have been stubborn, but I’ll report back.

Also no check engine light and no stored codes.
Do you have OEM size tires on the car? If you have bigger than normal tires your engine RPM might b lower than normal at 15 mph and the ECU is not operating the EGR.
Stock wheels and tires. The extent of my mods is tiny Haha.

So I had a really hard time with the removing the EGR, rounded a bolt, and any attempt to remove the nut failed. This is after soaking it in penetrating oil overnight. Tried heat, reverse sockets, and a bit splitter.

Gave up. So I removed the whole unit with the casing under it. Cleaned it as much as possible, and cleaned the pipe from the block. I’m ordering a new EGR unit, gaskets and nuts regardless at this point.
Thought I had sent the message above a week ago, sorry about that.

I installed a new EGR today, the old one was likely working fine though. I installed a new fuel filter as well.

I am planning on taking it to get smogged in the morning, so we’ll see what happens. I’ve also disabled the TCS.
Thought I had sent the message above a week ago, sorry about that.

I installed a new EGR today, the old one was likely working fine though. I installed a new fuel filter as well.

I am planning on taking it to get smogged in the morning, so we’ll see what happens. I’ve also disabled the TCS.
Have you checked to make sure the EGR ports on the intake manifold are clear? They get clogged full of soot and, even if you replace the EGR valve itself, exhaust gas may not be making it into the intake.
Yea I used a wire brush and got a bunch of gunk out. I also sprayed the intake manifold with seafoam, which has worked well for in the past with an old EG civic.

To be honest, at first when trying to clean the intake manifold side of carbon build up, it was hard to move the wire brush. Eventually it felt like there was an opening and the brush moved further inside the manifold, so I am hoping this does the trick.
I recognize that it is back together now; but, my thoughts are the same as @Honcho.

Flex-Hone makes some very small diameter hones.
The available diameters are in the correct range. Without seeing the actual product in the correct diameter I don't know whether the total shaft length is long enough or whether it is flexible enough to get through the EGR passage in the intake manifold.

Hopefully it passes and you are done.
So I passed with a ton of room to spare. So I think honestly the most progress was made with cleaning the ports of carbon and soot. I imagine I could clean even more if I wanted to. Given the car was not driven much over the last 30+ years, it makes sense as to why this could happen.

Thanks for all the feedback folks!