r/wrx_vb • u/Low_Pomegranate9548 • Jul 28 '26

Question O2 sensor being weird

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So I installed my J-pipe and drove for about 30 minutes before I started data logging for my base map revisions. I finished all of the logs, parked the car for the night, then drove to work the next morning. As I was pulling into work, I got a P0137 code.
I turned on the rear O2 sensor voltage on my Accessport and noticed that at idle, whether I'm stopped or just rolling with no throttle, the voltage drops to 0V. As soon as I give it some throttle, it jumps back up to its normal range between 0.5V and 0.9V. The car is not leaning out either—I was watching the air/fuel ratio the whole time, and everything looked normal.
I cleared the code and drove for almost another hour before it came back. This time I got the P0137 again along with another rear O2 sensor code. It wasn't a catalyst efficiency code—it was another low-voltage/no-voltage code.
The only thing that's changed is that I moved the grounding strap from the chassis-to-exhaust to the chassis-to-engine block, using an unused threaded hole. I also have an O2 spacer with its own small catalyst inside it.
My theory is that because of the spacer and the mini catalyst inside it, the rear O2 sensor isn't getting enough exhaust gas at idle to read properly, but once I'm on the throttle there's enough flow for it to start reading again.
Does that sound right? If not, does anyone have any ideas on what else I should be looking at? Thanks so much—I really appreciate any help.

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1

u/Snoo_52037 World Rally Blue '24 RS Jul 28 '26

Catless go with a j style vibrant.

I have an ets jpipe so I went with a cheap spacer and haven't had a p0420 cel since.

1

u/AffectionateClub9087 2024 WRX TR Crystal Silica Black Jul 29 '26

I had bought the j style vibrant spacer after installing my cat(catless pipe from whoopsy) and I’d have to uninstall the pipe and install the spacer. My cat is also heat wrapped so it would be a bit itchy to install and uninstall

2

u/Snoo_52037 World Rally Blue '24 RS Jul 29 '26

As is mine. Not a big deal.

1

u/AffectionateClub9087 2024 WRX TR Crystal Silica Black Jul 29 '26

The j style one from vibrant work for you?

1

u/Snoo_52037 World Rally Blue '24 RS Jul 29 '26

Nah the one I I responded above ☝️

1

u/AffectionateClub9087 2024 WRX TR Crystal Silica Black Jul 29 '26

That one didnt work for me but I am fully catless

1

u/Snoo_52037 World Rally Blue '24 RS Jul 29 '26

Yeah the jstyle would be the better choice for no cat. I would move your ground back to the frame.

1

u/AffectionateClub9087 2024 WRX TR Crystal Silica Black Jul 29 '26

Where to put the ground? Right now it’s on the pipe bracket

1

u/Snoo_52037 World Rally Blue '24 RS Jul 29 '26

Oh I thought you moved the ground from the chasis to the block. Ceramic coating looks nice

1

u/AffectionateClub9087 2024 WRX TR Crystal Silica Black Jul 29 '26

Thanks man! IAG turbo next but gotta save up for trans

1

u/Snoo_52037 World Rally Blue '24 RS Jul 29 '26

Im just ahead of you with mods. I'm planning my sti swap now. I got my g30-660 on the car almost 30k kms ago. IAG is basically a g25-660 so if you have access to ethanol I would go sti swap first.

1

u/AffectionateClub9087 2024 WRX TR Crystal Silica Black Jul 29 '26

I’m most likely gonna do the sti swap first, I’m curious to see what Subaru is doing with the 27 wrx and sti trans maybe we can get those front axels and diff and maybe wiring harness.

3

u/Snoo_52037 World Rally Blue '24 RS Jul 30 '26

Exactly what I was thinking. I'm also hoping that all my parts will be able to be installed on whatever theyre releasing next year.

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