Hey everyone, I have a '15 Forester XT with the dreaded P0420 code. Troubleshooting it is a little beyond my skill, but I did buy a code reader and took pictures of the data stream for both O2 sensors in hopes someone here might be able to guess what's up.
I bought the car just over a week ago, and the dealership filled it up with regular gas when they topped everything off. My hope is that the issue is simply the low-octane gas (especially since this is a turbo engine), however I can't be too sure. Yesterday I added some octane boost (the tank is still half full, so I haven't been able to fill up with premium gas yet) but only drove ~6 miles since then.
Symptoms:
- P0420 code
- CEL on, Hill Assist light on, cruise control disabled, S mode
- poor gas mileage (avg. 20 on highway, 16 in city)
- even at running temp the exhaust seems steamy (visible steam coming out of them), but I might just be overthinking it
- exhaust pipes have some soot in them. They were like this when I bought the car.
Other than the above the car runs great. No stuttering, it accelerates smoothly, etc.
Images of the O2 sensor voltage reads below.
Idling, with 1 instance where I revved the engine just a bit:
After clearing the DTC:
I have zero idea what a "normal" or "healthy" data stream would look like, so forgive me if nothing stands out. I'm hoping someone here can really just tell me if I should be alarmed by anything here, or if the data stream won't tell me anything and I need to keep troubleshooting.
Thank you!