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Check Engine Light issue

2 January 2017
Hi, hoping someone here can provide guidance diagnosing a check engine light (CEL) issue on a stock NSX with OBD1. Here's what I've observed and done so far.

When I saw the CEL lit up, I pulled aside the road to check the engine bay and exhaust areas and nothing seemed abnormal. I then turned off the car, waited for a few minutes, and started it up again. The CEL was off so I drove around and all seemed ok.

During the same day, I jumped the blue service connector port under the glove box to retrieve the codes. The CEL showed 4 long blinks, then 3 short blinks, and then 1 quick blink (and it repeats the same sequence). Here's a recorded video of the CEL blinks: https://streamable.com/y7fnti

I cleared the stored CEL codes by pulling out the clock fuse, waited for about 1 minute to reset, and reinserted the fuse back in. I started up the NSX and the CEL was off. So I went out for a drive for a few miles and no CEL, so I thought the issue resolved itself.

The next day I drove around and the CEL came back on again. I turned off the car wait a few minutes and started it back up again and the CEL was off. Drove around some more and the CEL lights up again. I noticed the CEL tends to light up when tachometer reaches around 5,000 rpm.

I pulled the CEL code again and it's the exact same blinking sequence as before. The NSX appears to drive fine (no idle/no engine hesitation issues) when the CEL is on or off. I'm trying to figure out what the issue is based on the CEL codes.

I reviewed the 'www.nsxprime.com/wiki/Trouble_Codes' page, and I think it's CEL code #43 and/or #1 ? Or is it only code #43 ? Can someone with expertise help confirm if I'm interpreting the CEL codes correctly?

What steps should I follow to diagnose and pinpoint what's causing this check engine light issue?
Have you had headers installed or replaced the front bank O2 sensor recently? i believe that is the cause for the 1 short blink code. A free way to test this i would guess is to swap the front and rear bank O2 sensors and see if the code switches from 1 short blink to 2 short blinks. Not an expert as im still learning the ins and outs of OBD1 CEL reading but it appears you have the code #43 and #1
I haven't touched or replaced the headers, O2 sensors, or exhaust (all stock). I'm new to troubleshooting the CEL, so any testing advice before replacing parts is appreciated. Do you know where the front and rear bank O2 sensors are located? Photos showing the location will help, thanks.
You are correct that your codes look like 43 (front fuel supply system) and 1 (front O2 sensor). There are some error conditions that have to occur 2 or more times before the CEL stays lit permanently. I did not think 43 was one of those codes; but, the service manual is not particularly clear on which codes have that multiple trip requirement. The multi trip requirement might explain why the CEL went off by itself when you stopped and restarted the engine.

If 43 occurs and the car drives just fine then combined with error code 1 that suggests that the problem is a front O2 sensor. The complete diagnostic procedure would be to use the test harness (I am guessing you probably do not have one of those) to measure the actual sensor voltages and confirm that the sensor has failed / is failing. In the absence of the test harness you can rig up a test using jumpers; but, that can be a bit fussy. You also need a good quality fast responding voltmeter with high input impedance to do the measurements. You can get .pdf copies of the service manual - fuel section which describes the voltage measurement procedures for diagnosing code 43 if you want to pursue that option. Given that you are also getting code 1 for the front O2 sensor it would not be completely unreasonable to assume that the problem is the O2 sensor and just proceed with replacement (I am normally not a fan of diagnosis by parts replacement).

The preceding is based upon the assumption that the car has no drivability issues. However, the fact that the problem does not seem to appear until around 5000 RPM makes me think that you might have a fuel delivery issue. This might only show up at higher engine loads. I suggest that you reset the ECU and then take the car for a test drive and and operate it so that it is entering VTEC operation - at least 6500 - 7000 RPM. Presuming that the codes have re emerged, check them to make sure that they are the same and that no new codes have appeared (particularly 44). If the car still drives fine and no new codes appear then its a good bet (but, no guarantee) that the problem is the O2 sensor.

My personal opinion is that I would not attempt to confirm the O2 sensor failure by doing a front to back switch. Aside from the fact that I think that the wire lengths are different so you may have a reach issue, if your O2 sensors are original they are at minimum 26 years old and may be a real treat to extract from the manifold. You may damage them on removal which would negate the test and at their age once it is out I would not want to be reinstalling an old sensor.

I believe Denso is the OEM supplier for the sensors and you can get aftermarket Denso sensors at a significantly lower cost than from the dealership. However, as I recall on the early cars there was a parts number mix up on the Denso part numbers. Larry B. had a long ago post with the correct part numbers so you might want to search for it to make sure you are getting the correct part numbers. NTK, Bosch and others also sell O2 sensors for the C30A; but, they sell the same sensor for the front and back which means one of the sensors will have too much wire which you will have to deal with.

If your 6500 - 7000 RPM test drive results in some drivability problems or you get some additional codes, then the problem may be beyond just an O2 sensor. You may have fuel delivery issues which could include a failing fuel pump, clogged fuel filter, clogged air filter or dirty injectors. Fuel pump, and clogged filters usually trigger errors on both the front and back cylinder banks.
I haven't touched or replaced the headers, O2 sensors, or exhaust (all stock). I'm new to troubleshooting the CEL, so any testing advice before replacing parts is appreciated. Do you know where the front and rear bank O2 sensors are located? Photos showing the location will help, thanks.

The O2 sensors are in the exhaust manifolds. Definitely an underneath the car job.

If you do a little searching on Prime, you will find a link to .pdf copies of the service manuals for both the 1991 car and the 1997-2005 'range'. The 1991 service manual does have some minor discrepancies relative to post 1991 cars; but, it is probably 98% valid. The service manuals are your best bet for diagrams showing the location of the O2 sensors and the other relevant info such as tightening torques.

My advice to anybody thinking about doing work on their NSX is to get the service manual. It will help you avoid potential worlds of hurt. I prefer paper manuals; but, the .pdf versions are free so no reason not to have one.
Thanks for the helpful info and suggestions. I've cleared the codes and drove it at VTEC RPM range and the same codes appear. The CEL usually comes on near or at VTEC, but sometimes the CEL is off. No performance issues at any RPM range. I'll review the service manual and research more on the O2 sensors. Hoping it's a simple fix.