My catalytic converters was going out in my 06 Nissan Xterra.

It has 112,000 miles. In this state a new catalytic converter is easily $1,000+

Screw that! I bought an 02 bypass sensor to put between the catalytic converter and 02 sensor.

My emissions code disappeared. Took my car for smog and it passed with flying colors.

Well I e lose some mpg. Makes sense because my oxygen sensor cannot detect how much oxygen is in the exhaust so it can’t tell the computer how much oxygen to mix in the fuel for fuel efficiency.

So my question is would switching to 91 octane fuel help bump up the mpg since my oxygen sensor can’t detect and signal to mix air/fuel accordingly?

No, higher octane means that it resists detonation better by slowing the flame front in the combustion chamber. If your car doesn’t have high compression/boost/advanced timing,etc, you’re not gonna benefit from it and will actually probably make less power, lower fuel economy, more pollution, less cash in your wallet from the unburnt fuel.

Regarding O2 sensors, does you car have a front and rear sensor? The front sensor upstream of the catalyctic converters is what the ECU uses for AFR. The second O2 sensor by the cat is purely for emmisions purpose. Those are commonly turned off to avoid codes when running a catless/high-flow cat in non-stock location and has no effect on actual engine performance.

Is it possible that your cats honeycomb broke off and is sitting sideways restricting your exhaust? That would cause a lost of MPGs.

You can buy universal cats pretty cheap, catco cats is what i have, carb’d truck, so no O2s

Universals not be California legal though.

Gut the cats or replace them immediatly!

You dont even want to know the damage bad cats do to the vq40 nissan engine.

Get rdy for dead engine.

Thenewx.org
Clubfrontier.org

Go read the horror stories about bad cats on the vq40.

Crap! Thanks for the reality check.

Move from CA to a free state

This. The cats can go bad and suck pieces back into the cylinders, destroying the engine. Honestly, I wouldn't even drive it until you can get them fixed.

I have 2 Xterras and am active over on thenewx.org.