Hey all, got a racing beat race connecting pipe and muffler for my '99. Trying to figure out a solution for my rear O2 sensor since I do not want to have it or the pigtail dangling in a bag next to the exhaust or have it affect my performance on the stock ECU I am currently running. Any write-ups or kits for this? Couldn't find any information about looping out the rear O2 sensor circuit. The racing beat race connecting pipe doesn't have a rear O2 bung for those wondering. Will be eventually going standalone but that is not currently in the budget, just need to figure out how to fool the OEM ecu right now or if it will even cause any issues besides a CEL (prefer no CEL if possible).

 

The 2nd sensor is there for monitoring the catalytic converter efficiency if not mistaken, so no adverse effects in disconnecting it (other than the CEL)

 

Years ago I remember a friend with a Spec Miata simply bent up a quick bracket, drilled a couple holes in it, bolted it to the firewall, and screwed the second O2 sensor into that. I believe that might solve your problem, so long as the sensor isn't touching anything. It'll show your cat as being very, very efficient.

 

Smog technician here. The downstream O2 sensor is primarily used for monitoring cat efficiency but it's also used as a fueling input to a very minor degree on modern vehicles. The ECU on your '99 NB may or may not take the downstream O2 data into account when making fueling adjustments, but even if it does, the ECU should disregard the input if it sees the sensor is disconnected.

 

In Australia, the second bung is simply plugged-off n NBs!

I've always understood its main purpose was to monitor the state of the catalytic converter so testers can do pre and post-cat comparisons.
For smog-testing they simply stick a sensor in the tailpipe; if you fail that, you need to go away and fix the problem, which is where you check the health of your catalytic converter (or undo all of your "modz" for re-testing).

 

Do you still have a cat? Is the car driven on public roads, and does a CEL matter for passing emissions where you live?

I have a CA-spec NB1, so the O2 sensor placement is a little different, bit I can confirm through experience that not running the downstream O2 has no impact on performance. I've been cat-less and downstream O2-less for something like 2 years of track use without issue - but the CEL is forever on. Doesn't much matter for a trailered race car.

If it's a street car, I'd honestly just get a bung welded in. You might be able to get some sort of sensor simulator from China, but that's violating federal law. If you don't have to worry about emissions, just zip tie the wire out of the way and put some tape over the CEL.

 

If you're running the stock ECU (for now) wouldn't the long-term fuel trim be affected with the second sensor just hanging in the breeze? In effect, the front sensor is controlling fuel and the ECU is seeing the rear sensor as being super lean all of the time and the ECU would try to compensate for that over time. It's just a guess.

FWIW, I'd get a second bung welded into the pipe post-cat and then plug it up when you get a stand-alone.

 

The long term fuel trim does get adjusted based on what the ECU sees over time, however, both fuel trims will mainly based on the upstream O2 and MAF sensor readings. The ECU on an OBDII-equipped car will also compare O2 sensor signals for correlation to try and determine if there is a fault with the other. I.E. if the downstream sensor is hanging in open air and the driver punches the gas causing a rich spike, the ECU will notice the front sensor going rich with no correlating response from the rear sensor. Done enough times, this will likely set a code and a check engine light.