P0030, P0031 and P0134 Scion FRS BRZ 86

This is a log of how I fixed P0030, P0031, and P0134 CEL codes after a UEL header install. Solution is at the end of this post.


April 2020
Brought car to a shop to install Gruppe-S UEL Headers
OFT Tuned for Stage 2+
Ran for a month no issues


After said month, felt loss of power, thought I just sucked at driving manual
CEL Popped after driving 1-2 hours
Scanned the code for P0030 And P0134.

Let it sit overnight, cleared the codes and it drove normally (no loss in power).
Drove for another hour, CEL popped again.

Attempted fix

Tried to reseat the O2 sensor (right side/gray connector)
Replaced O2 sensor (DENSO 2349136 {#22641AA640, 22641AA66A, SU00300425}).
Cleared the CEL and drove it for a week no issues (though trips were <1 hour)

Drove 1.5 hours, CEL popped.
Checked the fuse according to this thread.
Checked the EFI(HTR) and it was fine. No issues based on visual inspection.

Follow up #1

Near the end of the 1.5 hour trip mentioned above, a loud sound could be heard on deceleration and/or hitting regular road bumps (non potholes. Think line-fillers).

I initially thought it was it was my front lip falling off on one side and scraping the road. Stopped the car, checked it out, nothing was scraping. It sounded like sand paper against concrete, or a front lip scraping the road.

Follow up #2

Today I took off the underbody panel and saw this (i didnt install the header; took it to a shop).

Checked for exhaust leaks, didn’t feel any air on hand. Checked O2S1 Voltage Deltas and got this:

Follow up #4

I left it be for a while and took it for a long drive today (40m-1h). Kept monitoring the 02 Sensor voltage to make sure it was updating. No issues. Will report back if the codes come back.

Follow up #5: Final

So during the course of 1.5 weeks, I drove very little. Every trip resulted in a CEL with the exact same codes. Reflashed the ROM with no success.

Decided to spray the contacts with electrical cleaner and plugged it back in. Here is the real kicker: I tugged on it to see if it was snug – it was not. I tried to reseat, with no success. I grabbed pliers and pinched the connectors together and heard a click.

Drove around for another hour and a half – P0171 – High airflow/imbalanced AFR. Engine was definitely bogging and this is most likely the ECU retuning to live sensors finally reporting data. Let it cool down and tried again. Solid. Rock solid.


Make sure the connector is connected snug with the clip engaged. I believe the mech who installed the headers and therefore the O2 sensors didn’t plug it in all the way and cleared the contacts causing intermittent issues. Every time would plug them in would reengage the contacts, but not securely resulting in the repeated codes.

Use angle grips to press the connectors together.