prwood Posted September 10, 2015 Share Posted September 10, 2015 I'm trying to diagnose a pending P0420 code on my 2002 Subaru Legacy. Right now I'm using an OBDII scan tool to read the various sensors and I had a few questions. First, I have two oxygen sensors listed: O2B1S1 and O2B1S2. Is S1 the upstream/pre-cat and S2 the downstream/post-cat? Second, from what I've read, when monitoring these sensors, I should expect the upstream sensor reading to generally fluctuate while the downstream sensor reading generally stays constant. Is that correct? Thanks! Quote Link to comment Share on other sites More sharing options...
swootton Posted September 10, 2015 Share Posted September 10, 2015 Both the upstream and downstream sensors should fluctuate. The forward sensor should sweep widely .2-.8V and the rear sensor should cahge just as rapidly but only sweep .35-.65V. Most likely you need a cat. Never has a Subaru with a 420 that was fixed with just o2 sensors. BTW, I am a general manager at a repair facility and we do a lot od Subaru work. Quote Link to comment Share on other sites More sharing options...
ocei77 Posted September 10, 2015 Share Posted September 10, 2015 Not a cat. 99% of 420 issues are resolved with front O2 sensor replacement. Do a search for Subaru dtc codes and your checklist will be shown. O. Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted September 10, 2015 Share Posted September 10, 2015 first, clear the code and see if/how quickly it returns. I would never proceed too quickly on a a spurious pending code. double check for exhaust leaks - pre-cat. on a 2002, that code is not used to adjust any operating parameters. not that it may not indicate something that needs attention, just that, it's sole function is to say it thinks the cat conv. isn't working. Some people with otherwise perfectly running cars cheat the rear sensor with a resistor or an extension pipe (sparkplug extender w'ever) to avoid that code. Quote Link to comment Share on other sites More sharing options...
prwood Posted September 10, 2015 Author Share Posted September 10, 2015 First thing to note is that this car is due for a MA state inspection, which will require a clean report from the OBD. I am waiting to get an OK report on CAT and EVAP before I take it in, but all other systems report ok. I've reset the codes several times, and each time the p0420 comes up as a pending code. It has not yet triggered the CEL, however. Currently, when I start up idling and bring the ECT to 188, o2b1s1 generally hangs around 1.5 while o2b1s2 hangs around 0.08-0.10. Any thoughts on those values? Since it hasn't come up as a permanent code, I will try doing some more driving and see if it goes away or comes to stay. Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted September 10, 2015 Share Posted September 10, 2015 you try one of those extenders, if it prevents the code, go get your test, then continue troubleshooting. http://www.amazon.com/Blox-Racing-BXFL-00108-Eliminator-Adapter/dp/B000W1XX4M/ref=sr_1_fkmr1_1?ie=UTF8&qid=1441903416&sr=8-1-fkmr1&keywords=rear+o2+sensor+extender Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 10, 2015 Share Posted September 10, 2015 A complete tune up is a good first start - stuff that should be up to date anyway. Plugs, wires, air filter, PCV valve. Where are they? Exhaust leaks are not good. Take care of those. I generally replace front O2 sensors just simply because I don't plan on the original sensor lasting 300,000 miles so I might as well replace it preemptively rather than wait for symptoms and compromised gas mileage. It will usually come to stay - google P0420 - it's an awful annoyance. Often they're very tricky to diagnose. Mechanics usually just install a new converter and call it good. It's also meaningless in 2002's - it's not used by the ECU. you can cut the rear O2 sensor off and the car will run completely 100% fine for the life of the vehicle and can still pass the sniff test (though they won't test it with the code showing obviously). for that same reason - i'd install an O2 extender or resistor to just trick it. gas mileage is a better litmus test as to how well the engine is running. poor mileage = fix your car. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 10, 2015 Share Posted September 10, 2015 +1 - buying the extender will buy you time even if you don't like that as a permanent solution so you can properly trouble shoot too without it being a debacle on your hands. Quote Link to comment Share on other sites More sharing options...
prwood Posted September 10, 2015 Author Share Posted September 10, 2015 First thing to note is that this car is due for a MA state inspection, which will require a clean report from the OBD. I am waiting to get an OK report on CAT and EVAP before I take it in, but all other systems report ok. I've reset the codes several times, and each time the p0420 comes up as a pending code. It has not yet triggered the CEL, however. Currently, when I start up idling and bring the ECT to 188, o2b1s1 generally hangs around 1.5 while o2b1s2 hangs around 0.08-0.10. Any thoughts on those values? Since it hasn't come up as a permanent code, I will try doing some more driving and see if it goes away or comes to stay. FYI: I went out for another drive, putting the engine through its paces, and the o2b1s1 kept hanging pretty close to 1.5 while the o2b1s2 was all over the place. Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted September 10, 2015 Share Posted September 10, 2015 there are youtube videos showing how to use an infrared remote thermometer to measure exhaust tubing pre and post cat. If you feel the cat is working, you might try a new rear O2 sensor. Quote Link to comment Share on other sites More sharing options...
prwood Posted September 10, 2015 Author Share Posted September 10, 2015 A complete tune up is a good first start - stuff that should be up to date anyway. Plugs, wires, air filter, PCV valve. Where are they? Just replaced spark plugs last week. Replaced wires last year; I disconnected them and visually inspected when changing spark plugs, they looked good. Air filter was replaced last week. I pulled the PCV valve and checked it last week and it looked/sounded good. It had been replaced last year anyway. I had the battery disconnected while working on the spark plugs. If the check engine light does come on before the ready tests are all clear, then I'll look into getting the extender. It's still not on yet after I took it out for a 45 minute drive earlier today and the EVAP and CAT systems are still not showing ready, and the P0420 still remains in a pending state. I've taken it on several 30-60 minute drives earlier this week, about 50/50 highway/city driving. Quote Link to comment Share on other sites More sharing options...
brus brother Posted September 10, 2015 Share Posted September 10, 2015 I have been running mine with p0420 for at least 4 years. Clear the pending code and then do the runaround to set the systems to get it pass inspections for another year. I will try and search out the exact procedure. I know I have it linked somewhere. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted September 10, 2015 Share Posted September 10, 2015 I have been running mine with p0420 for at least 4 years. +1 my 2002 OBW has had it since I've owned the car over 100,000 miles ago - 5 or 6 years. Oddly it went off for a few months earlier this year..!??!!?? What!?!? Odd since it's literally never been off for like 5 years. Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted September 10, 2015 Share Posted September 10, 2015 +1 my 2002 OBW has had it since I've owned the car over 100,000 miles ago - 5 or 6 years. Oddly it went off for a few months earlier this year..!??!!?? What!?!? you need the reverse-extender. Quote Link to comment Share on other sites More sharing options...
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