Hello, I am new to forums, so I apologize in advance, for any transgressions I may make. My granddaughter has a 2017 Outback 2.5 Limited. Approximately 135,000 miles. About 2 weeks ago, the ECU tossed a P0420 error. Scanned it and found 3 instances, 1 permanent. We recorded livestream data on both O2 sensors, as well as the STFM and LTFM. Recorded on fully warmed up engine at both idle and 2500 rpm. Values for the downstream O2 sensor were consistently in the .735-.740 range at both rpm levels. STFM readings were consistently within +/- 5, mostly close to 0. LTFM readings were consistently 0 on 12 frames. STFM ranged close to 0 on 12 frames.
I did some research on these readings, and I am given to understand that the fuel trim readings indicate no vacuum leak or exhaust leak, and this points to the downstream O2 sensor. Even though the sensor readings do not indicate it is failing, I learned that even so, it may intermittently throw an error. It also seems that aftermarket sensors are not recommended for Subarus. I ordered a Denso sensor, as I found that Denso supplies Subaru with 'OEM' sensors. So, I felt comfortable using Denso. We replaced the sensor and, after a few drive cycles, the check engine light went out on its own and all codes, including the permanent code were cleared by the ECU. This told me that the ECU considered the problem solved. However, a couple of days later the P0420 code is back, with 1 permanent and two that can be cleared.
May we please have some help? BTW, I will be 80 on my next birthday. So, if I have done anything dumb here, please consider this.
Thanks to all!
I did some research on these readings, and I am given to understand that the fuel trim readings indicate no vacuum leak or exhaust leak, and this points to the downstream O2 sensor. Even though the sensor readings do not indicate it is failing, I learned that even so, it may intermittently throw an error. It also seems that aftermarket sensors are not recommended for Subarus. I ordered a Denso sensor, as I found that Denso supplies Subaru with 'OEM' sensors. So, I felt comfortable using Denso. We replaced the sensor and, after a few drive cycles, the check engine light went out on its own and all codes, including the permanent code were cleared by the ECU. This told me that the ECU considered the problem solved. However, a couple of days later the P0420 code is back, with 1 permanent and two that can be cleared.
May we please have some help? BTW, I will be 80 on my next birthday. So, if I have done anything dumb here, please consider this.
Thanks to all!