After swapping the O2 sensor, I am still getting P0134 CELs, as well as P0030 and P0031.
It’s been a few months since experiencing this issue. I replaced the overpipe gasket in order to resolve an exhaust leak. As per many other articles online, it is VERY possible for this code to trigger when an exhaust leak is existent.
Removing the header to gasket bolts, and cleaning up all the gasket maker that the mechanic has applied, we see a small spot that is carbon build up.
Inspecting the gasket, it’s clear they reused the gasket and hoped that the gasket maker would hold up.
The Gruppe-S gasket is shown on top, and the reused OEM overpipe gasket on the bottom. The carbon build up is even clearer here. The gasket maker was essentially blown out and caused a small gap.
The fix is simple. Either install the included gasket provided by Gruppe-S or visit your local Subaru dealership and request an Overpipe gasket (Part number: 44011AG000).
Additional troubleshooting steps
If you want more troubleshooting steps for P0134, P0031, or P0030 for your 86, BRZ, or FR-S, keep reading.
Trace the wiring
This was a difficult troubleshooting step for me, but I figured it all out looking at the manual.
Step 1: Confirm code behaviour
To trip this code, follow the driving pattern TWICE, with an engine restart in between each run. It will trip for sure. Clear the code before the initial driving pattern.
Step 2: Test the electrical source.
Disconnect the A/F O2 sensor (gray). Get a multimeter and measure C14-2 to ground. This is the second pin on the top row.
Test the ECU cable/wiring harness
Remove the glove box, and you should see the ECU.
There are 4 cable bundles. Unplug the top two. The second one from the top is what you’ll need, but unplugging the top one would make it easier.
With a multimeter, clip to pin 3 of the A/F sensor receptacle as per step 2. On the other end, test the resistance of pin 17, 18, and 19. Only pin 19 should yield a reading. Otherwise, you have a short or a damaged cable (that is if pin 19 to pin 3 do not yield a reading).
Pin 3 of the sensor should coincide with pin 19 of the A34 connector.
Pin 4 should be pin 18 of the A34 connector
They should measure <1ohm of resistance.
Since I was able to confirm 0.7ohms of resistance, an electrical/wiring fault was ruled out. The next option according to the manual is to purchase an ECU. Evidently that would’ve been an expensive waste of money.