gwicks Posted January 19, 2016 Share Posted January 19, 2016 Hi everyone, I just moved to the USA from Australia, and will be living here for the next 4 years. I just purchased an 06 legacy 2.5i with 116k miles on the clock. As soon as I got it I had the head pulled off and machined, replaced all timing belt, head gaskets cam and crank seals, spark plugs etc, as I would like to keep the car until I leave, i doubt i will clock up more than 50k miles during my stay. About a month after purchase I started to get P0420 codes, I have a bluetooth OBDII scanner and the torque pro app, and have just been clearing the code my self. I am trying to diagnose the problem myself, and I know i need to look at the data from both the o2 sensors to see if the rear sensor is fluctuating like the front, my problem is I can't seem to find more than one o2 sensor showing data. O2 1x1 has no information to display, O2 1x2 shows the O2 voltage fluctuating rapidly between .2 and .9V. So my question is, is O2 1x2 definitely the rear o2 sensor, and what should i be looking at to get the front o2 sensor information, AFR© or AFR(m) or something like that? Or if 1x2 is the rear sensor do the fluctuations definitely mean my cat is bad? Sorry if this question has been asked before, i have started reading some of the massive 85 page + threads about this issue, and whilst they clearly explain how to diagnose the problem, i have not found information about which info i should be looking at for the front 02 sensor, most posts assume o2 1x1 and 02 1x2 is the data that should be showing. (all the youtube videos for example show graphs of those readouts) Thanks for any help, this code is quite annoying. On the plus side, drove the legacy up to mt hood for 3 days over the long weekend, and she drove like she was on rails over some pretty deep fresh snow and ice!! Link to comment Share on other sites More sharing options...
apexi Posted January 20, 2016 Share Posted January 20, 2016 I'm gonna base this reply off the factory service manual, since I haven't had a chance to see if my elm327 wifi obd2 reader still works with my android phone I got a month ago. The factory service manual makes it seem like if your rear oxygen sensor values are fluctuating a lot, then the sensor is bad. The attached images make it seem like the rear oxygen sensor readings should be fairly stable regardless of whether you're driving or idling. As far as whether you're looking at the correct oxygen sensor, I'm not sure about that. From what I have read on here, p0420 means either the rear oxygen sensor is bad, or your catalytic converter is bad. Rear oxygen sensor is cheaper than a catalytic converter, so I would probably just replace the sensor and see what happens. Link to comment Share on other sites More sharing options...
gwicks Posted January 20, 2016 Author Share Posted January 20, 2016 Thanks for the info apexi! Assuming o2 1X2 is the rear sensor, i would say my cat is definitely wrecked. Do you think it's worth replacing? I was happy to pay the grand for new head gaskets and timing belts because i didn't want the car to die, but from what I understand, the cat's have no effect on the running of the car, just the emissions? If worth replacing, does anybody have any recommendations as to aftermarket, direct bolt on replacements? The OEM ones seams stupidly expensive. Thanks again for your help mate!! Link to comment Share on other sites More sharing options...
apexi Posted January 20, 2016 Share Posted January 20, 2016 What state are you in? PZEV Subaru's have a 15yr, 150k emissions warranty, so you could get that catalytic converter replaced for free. Link to comment Share on other sites More sharing options...
gwicks Posted January 20, 2016 Author Share Posted January 20, 2016 I'm in Oregon, and from the CARFAX report i got when before i bought the car it looks like it has spent it's entire life here. (Not surprising, subaru's seem to make up 50% of the cars here, at at the snow last weekend it was more like 80%!) Link to comment Share on other sites More sharing options...
gwicks Posted January 20, 2016 Author Share Posted January 20, 2016 OK did some more testing today, at a steady 2500 RPM the rear o2 sensor stays pretty much flat at 0.8V, got under the car with a laser tempertature gun, and the back of the cats seams to be hotter than the front of the cats, so now i'm thinking it can't be the cats. Link to comment Share on other sites More sharing options...
gwicks Posted January 20, 2016 Author Share Posted January 20, 2016 Ok went for another drive with logging enabled, and got some readings. The front wide range voltage o2 sensor is usually around 2.8V, but will occasionally jump to 5.5V, and around 0.2 seconds later the rear o2 sensor will drop to 0.0v. This happens at a fixed RPM. Link to comment Share on other sites More sharing options...
gwicks Posted January 21, 2016 Author Share Posted January 21, 2016 pic of wideband and sensor 2 attached Link to comment Share on other sites More sharing options...
Juventas Posted February 9, 2016 Share Posted February 9, 2016 I also get no data on O2 1x1. Google says that a wide range O2 sensor is a newer, more sophisticated version of a traditional O2 sensor. Not sure how this effects whether we can use matching fluctuations as a way to diagnose a bad cat. Link to comment Share on other sites More sharing options...
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