citrus

Hello everyone, bone stock '14 MR here.

Computer threw a P0138 today and nothing else. The exhaust smelled fine and the engine pulled fine (up to about 5500 RPM, didn't go near WOT), so I cleared the code. I took the car on two brief drives afterwards and the code didn't come back. Hopefully, it stays that way.

I asked around in my local Evo group and searched as much as I could online, but I found conflicting opinions. On one hand, the trouble code dictionaries all say that not addressing P0138 can lead to catastrophic cat and/or engine failure. On the other hand, all the owners say the rear O2 sensor isn't really necessary, so wiping the code is fine.

Keep in mind our car is completely stock, and it's a grocery-getter.

What does rear O2 sensor actually do? What are the consequences of ignoring it? What could've caused the P0138 in the first place?

Thank you for all your help!

splattj

iTrader: (2)

The rear O2 is primarily used to monitor the efficiency of the Cat whereas the front is for fuel trim. My '11 Ralliart threw a P0135 and replacing the sensor fixed it. Ignoring it shouldn't have any real consequence aside from a check engine light. Though, if replacing it doesn't fix the issue, it's possible that your cat-con failed which, if clogged up, could cause you some loss of power/backpressure issues with your exhaust.

xerxess

citrus

Thanks for the response!

The code came up after I picked up the car from window tinting. I imagine the car was cold when it came out of the shop. The weather was pretty warm that day, easily t-shirt weather. I cleared the code and it hasn't come back after two trips.

I think the code may have been triggered from me messing around fuses the day before. I was inspecting my relays, and I unplugged and reseated the O2 sensor relays. Could that have done something, maybe?