My idle has began fluctuating up and down by about 50-100 RPM and the AFR's will swing wildly between 10:1 and 20:1. This only happens when the car is fully warmed up. AC/defroster is off. At cold or semi-warm idle everything is fine. I put the car on my lift while running and I do not hear any exhaust leaks or see any soot that would indicate a leak. The torque for all exhaust fasteners was checked once a week for about 4 weeks after installation to make sure they were still tight. I'm assuming it's an O2 sensor issue since most cars don't use the O2 sensor until fully warm, correct?
Log of 100% warmed up idle:
https://datazap.me/u/jmtrotz/2015-fr...og=0&data=1-13
Log of semi-warm idle, going for a drive, and ending with 100% warmed up idle:
https://datazap.me/u/jmtrotz/2015-fr...og=0&data=1-13
Car details:
2015 FR-S
Shell 93 octane gasoline
Gruppe-S header/over pipe with GrimmSpeed gaskets
Stock front pipe
HKS Legamax catback
OFT Stage 2 UEL 93 v2.076 off the shelf tune
Stock intake/airbox