I have an 05 wrx. After installing a txs rfl tbe with high flow cat. i am gettin the p0138 code and 0420. I know the 0420 is just cat effeciency below threshold, and i know the 0138 is sensor high output or something along those lines. The only other mods are a bov. My question is what is 0138? Do I need a new 02 Sensor or just need the fix?

thank you for any help in advance.