urban_sub Posted July 22, 2009 Share Posted July 22, 2009 This thread will probably be ignored since it involves the dreaded P0420. Well here's my run on this. 1996 Legacy AWD sedan with 2.2L. Approximately 150K miles and been quite well cared for. Did my searches and read up on the possible fixes to this issue (but there are so many threads on this). So I have done thus far: 1) Changed the O2 sensors (both) 2) Checked for exhaust leaks 3) Changed the Cat 4) Changed the MAF 5) Tune-up (plugs, wires and filters) 6) Injector cleaners 7) Clean throttle body internals and I still have the P0420. Voltage test of the O2 sensors show that there is allot of fluxuation on the pre Cat. This made me think the problem is either upstream or the sensor itself so I changed the sensor again and did all the engine items but no help. Lately I've been clearing the code when ever it comes up but it eventually returns within 300 miles. Any recommendation to what I should do from here? Hard to junk the car when everything else is fine. Runs great - its just that CEL lite. Quote Link to comment Share on other sites More sharing options...
Log1call Posted July 22, 2009 Share Posted July 22, 2009 Not using fuel additives? Engine sealers are all cat/O2 approved? Exhaust is getting hot enough? That code is fairly specific and not prone to being something else if it's by itself. It compares the before and after readings and expects the second O2 sensors readings to range more slowly from above mid point to below mid point of their range. It is not comparing how far but how fast they range, so it's fairly fool-proof. i'd suspect something is polluting your cat. Quote Link to comment Share on other sites More sharing options...
OB99W Posted July 22, 2009 Share Posted July 22, 2009 (edited) See this article for some additional ideas: http://www.catalyticconverter.org/news/news_page.cfm?Key=catalytic_converter-&News=120 Edited July 22, 2009 by OB99W Quote Link to comment Share on other sites More sharing options...
avk Posted July 22, 2009 Share Posted July 22, 2009 At this point, ECU and wiring might be suspect as well. Quote Link to comment Share on other sites More sharing options...
Log1call Posted July 22, 2009 Share Posted July 22, 2009 Good link. Can we get that stickied or added to the FAQ data base somehow? Quote Link to comment Share on other sites More sharing options...
nipper Posted July 22, 2009 Share Posted July 22, 2009 It is possible that your cat is bad. On occasion a 420 is a bad cat. Quote Link to comment Share on other sites More sharing options...
Log1call Posted July 22, 2009 Share Posted July 22, 2009 Have you had it on a obd scanner? It should/could have recorded the engine operating conditions at the moment that code was set. That should help pin-point whether it was too rich at the time, if the evap systems were malfunctioning etc. Quote Link to comment Share on other sites More sharing options...
StructEngineer Posted July 22, 2009 Share Posted July 22, 2009 What sort of cat and O2 sensors did you use? Subaru's are picky. Are you absolutely sure there's no exhaust leaks? How's the fuel mileage? When you say there's fluctuation in the primary o2, do you mean more movement than the normal oscillation that supposed to be there? Quote Link to comment Share on other sites More sharing options...
urban_sub Posted July 22, 2009 Author Share Posted July 22, 2009 Thanks for all the responses. I can always depend upon the Suby community for some good feedback. Lets see how to answer: See this article for some additional ideas:http://www.catalyticconverter.org/news/news_page.cfm?Key=catalytic_converter-&News=120 Thanks for the link OB99W. This is good. I went over it a couple of times real quick and it seems I applied all the ones for my case but I will look through it with more detail later. Not using fuel additives?Engine sealers are all cat/O2 approved? Exhaust is getting hot enough? That code is fairly specific and not prone to being something else if it's by itself. It compares the before and after readings and expects the second O2 sensors readings to range more slowly from above mid point to below mid point of their range. It is not comparing how far but how fast they range, so it's fairly fool-proof. i'd suspect something is polluting your cat. Have you had it on a obd scanner? It should/could have recorded the engine operating conditions at the moment that code was set. That should help pin-point whether it was too rich at the time, if the evap systems were malfunctioning etc. Log1call - I think this could be the case as all the usual fixes are not helping. But the MPG is not too off – avg 22-23 MPG. Was getting 24 before but there has been a significant change in driving conditions too (more city and less highway). My OBD scanner just has code reading and clearing features. I have to upgrade SW to get all the advanced features but I can’t afford it right now. At this point, ECU and wiring might be suspect as well. You are scaring me but yeah that can be a possibility. ECUs are pretty expensive – right? It is possible that your cat is bad. On occasion a 420 is a bad cat. Nipper -I had the cat changed already. Not unless the shop that did the leak check and cat change stiffed me. What sort of cat and O2 sensors did you use? Subaru's are picky. Are you absolutely sure there's no exhaust leaks? How's the fuel mileage? When you say there's fluctuation in the primary o2, do you mean more movement than the normal oscillation that supposed to be there? O2 are the exact same Bosch part number as the ones replaced. The cat is an aftermarket from the cheapest quoted shop that also said that they did a exhaust leak check. This part is the only thing I didn’t do personally. I am starting to question it now. As much time and effort I have placed on this issue I should have brought it to a reputable Suby shop and had them handle it. The Pre O2 sensor is sweeping the range about twice a second when the RPMs are steady. I thought it should be more like once every second or two. Well all this will keep me thinking for a while. If there is anything else anyone can think of let me know. I'll update as I try or retry some of the things mentioned here. Maybe first rip open the heat shield over the cat and see if they truly changed it. Thanks again. Quote Link to comment Share on other sites More sharing options...
avk Posted July 22, 2009 Share Posted July 22, 2009 (edited) You are scaring me but yeah that can be a possibility. ECUs are pretty expensive – right? An OE remanufactured unit for my friend's 2000 Outback was around $200. It wasn't the problem though. I posted a thread similar to yours, about a P1133 code that wouldn't go away. We unwrapped the harness and began tracing individual wires, but in the process the problem did go away, apparently after unplugging and plugging back some connectors. Edited July 22, 2009 by avk Quote Link to comment Share on other sites More sharing options...
subaru360 Posted July 22, 2009 Share Posted July 22, 2009 What kind of cat? You will still get a 420 code if you used an aftermarket cat. If that's the case you can try the anti-fouler trick. Buy 2 spark plug anti foulers. Drill the center of the first one out and put them both on the rear O2 sensor. This usually works. Quote Link to comment Share on other sites More sharing options...
Bucky92 Posted July 22, 2009 Share Posted July 22, 2009 My Legacy tosses a 420 code everytime it rains. I just carry my OBDII reader with me and have gotten good at clearing it on the fly. Once cleared its good for about 2 weeks. So the comments about paying attention to the conditions when it comes on is important... the only reason I havent looked into mine further is cause.. my Legacy is the beater car..not spending much time, effort, or money on it. Quote Link to comment Share on other sites More sharing options...
Log1call Posted July 23, 2009 Share Posted July 23, 2009 Free software here... http://www.obd2allinone.com/sc/pages.asp?pageid=60 The O2 sensors have different frequencies in different vehicles, because of both design and state of tune. The faster and the smaller the change in voltage the better but it's not critical; several times a minute is often quoted as minimum in manuals. Quote Link to comment Share on other sites More sharing options...
StructEngineer Posted July 27, 2009 Share Posted July 27, 2009 The cat is an aftermarket from the cheapest quoted shop that also said that they did a exhaust leak check. This part is the only thing I didn’t do personally. I am starting to question it now. If you put the cheapest possible cat on I think that is your problem. Most foreign cars are picky about aftermarket cats. If you go with el cheapo it's not going to smooth out the secondary o2 readings enough for your ECU to be happy. Quote Link to comment Share on other sites More sharing options...
nipper Posted July 27, 2009 Share Posted July 27, 2009 cheapest = Eastern ? Eastern BAD nipper Quote Link to comment Share on other sites More sharing options...
unibrook Posted July 27, 2009 Share Posted July 27, 2009 Is there an EGR valve on that puppy? Has that already been replaced? Quote Link to comment Share on other sites More sharing options...
urban_sub Posted February 7, 2010 Author Share Posted February 7, 2010 (edited) Update - Problem Solved Sorry this took so long but I just kept putting this issue off. Found that I got stiffed on the Cat. After removing the heat shield, I found the original cat was still in place and the shop just spray painted it and replaced the heat shield (much too late to take issue with the shop that did the job two years ago). A Suby Indy did some test and found the Post O2 sensor out of spec. Replaced the sensor with another Bosch one and ran for a couple months. The code has not returned so I say problem solved. Thanks everyone for the advice. I guess I could have saved $$$ if I had done some proper testing in this process. I was leaning towards a minor exhaust leak that I could not detect. Who would have thought a Bosch sensor was bad right out of the box. Well I am just Happy it is back to normal. Edited February 7, 2010 by urban_sub Quote Link to comment Share on other sites More sharing options...
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