I have had my old 2005 Nissan Pathfinder for a while now, probably 2015 or so. Been through a lot with the old dog. For the last two years I have kept it to be just a towing rig for my boat. It gets about 1000 miles a year now. Hoping to keep it going for one more summer.

This spring, the exhaust was blowing a little air by somewhere intermittently, then it got louder, figured it was the crack where the passenger front cat flange had separated from the pipe that I had sealed up with some exhaust sealant paste last summer.

When it got louder my gas milage got better by about 3mpg and the p0174 light went away!

Got under there today and and the cheater spacer I had on the passenger side downstream o2 sensor had rusted away and the o2 sensor was dangling. This happened on the drives side two years ago and I re did it, which was a huge pain in the butt. The crack with the sealant actually held up well. I put some more on while I was under there.

Well, I just tucked the O2 sensor up behind a heat shield and filled the cheater opening with the exhaust sealant.

We will see how long it holds!

After dealing with this car's CATs, O2 sensors, engine lights, radiator bypass, cam position sensor, you name it, I am about done and satisfied with minimal effort.

What do you think, run it and don't look back?

(garage will pass it for inspection)

I dunno what your miles are, but your mpg shooting way up with a hole in the exhaust implies that your exhaust is partially blocked. Since a leak at the O2 sensor helped, it's probably one or both rear cats. Evidently you're in a non-inspection state, so straight-piping them is an option.

Could be running leaner as well, but plugged up exhaust seems more likely.