FRaMist Posted October 12, 2015 Share Posted October 12, 2015 Greetings! I think I've figured out where to post this on the forum.I am a brand new Subaru owner. 1997 Legacy Outback Wagon with standard engine. I love it. 232K miles, and two codes for the SEL prevent me from passing inspection in NJP 0325 Knock Sensor Circuit P 0420 Cat Efficiency Bank 1 The knock sensor is seems fairly straight forward. I will replace it and hope the code goes away.At first, I thought I would have to replace the whole catalytic converter...but I got advice here NOT to do that. The mechanic I hope to trust here said that the P 0420 (which he knew off the top of his head) would NOT BE CLEARED by replacing the O2 sensors as a few experts here previously suggested. He says a flush of the cat system would clear the code temporarily, but that the CAT EFFICIENCY reading implies that the two sensors are working, and the cat is clogged.His price is around $170 for chemicals and labor and he thinks that the code will clear in order to pass inspection.Advice is welcome. I like the idea of the price for the one year of inspection, I'll prove that the O2 sensors work and save up for a new Cat system later. However, with the knock sensor code, it does seem interrelated according to postings here. I don't know which road to travel? Nearly broke, but must get the car passed the NJ state inspection system which is entirely code-based.Sincerely! Erik Link to comment Share on other sites More sharing options...
DaveT Posted October 12, 2015 Share Posted October 12, 2015 I have had 2 battles with the p0420 code on my 01 Forester. Sensors did not help. New cat solved it one time. The other time, one of the brackets had snapped, and rubbed a small hole in the pipe. Patched the hole, and cleared the codes. Passed. 1 Link to comment Share on other sites More sharing options...
lstevens76 Posted October 12, 2015 Share Posted October 12, 2015 Did this mechanic hook up a live data scanner and see what the cats are doing? Has the code been cleared to see if it returns and when it returns? A P0420 refers to a Catalyst Inefficiency. What this actually means is that the readings between the front 02 Sensor and the Rear 02 sensor differ enough to make the computer believe the cat is not working correctly. A bad rear 02 sensor that is "within spec" but not providing correct readings can trip this code. A front 02 sensor with the same issue as above can cause the same thing. A bad catalytic converter can cause this. An engine running way to rich or way to lean can also produce this on older vehicles due to catalytic wear. This doesn't mean the cat is done, it means there are pre-existing problems that are causing the issue. If he didn't "diagnose" the problem and just read the codes, this mechanic doesn't know. Replace the knock sensor and clear the codes and see what comes back. That is the first step. You might also try a tank or two of premium fuel. 1 Link to comment Share on other sites More sharing options...
AdventureSubaru Posted October 13, 2015 Share Posted October 13, 2015 There is a very long list of things that can throw the p0420 code. Rarely is it ever the cat itself. Replacing the cat is a last resort. As mentioned, see what happens after a new knock sensor is installed. Bad knock sensors can make the motor all kinds of goofy sometimes. Link to comment Share on other sites More sharing options...
idosubaru Posted October 13, 2015 Share Posted October 13, 2015 Ideally you replace the knock sensor and then see if the P0420 goes away. The sensors are like $10 on ebay and requires removing ONE BOLT to replace, super easy, anyone can do it. This code is really tough to diagnose and most people (even mechanics) don't know how to diagnosis it or it's not very efficient time wise. It's nice to verify for certain the converter is bad - if something caused it to fail, that might cause the next one to fail. Unlikely on a car we know something about but if you just bought this one there's a lot unknown. Catalytic converters generally last the life of the vehicle on Subarus, they are robust and failure is far less common than you would guess by the way people throw around the phrase "clogged converter" - i've never even seen that happen on an OEM converter in 20+ years of working on Subarus. A tune up would be wise - but if you have no cash you're not going to want to do that. Install a rear O2 spacer for $5 and see if it goes away. It doesn't always work but it works often enough it's definitely worth $5 and 15 minutes to install. these are just examples, there's probably cheaper ones: http://www.amazon.com/O2-Sensor-Spacer-Adapter-Extender/dp/B00CZFCPQU http://www.ebay.com/itm/like/261582980274?ul_noapp=true&chn=ps&lpid=82 Link to comment Share on other sites More sharing options...
FRaMist Posted October 15, 2015 Author Share Posted October 15, 2015 Thanks! Excellent advice!Answers: No, a live data scanner was not used on the cats. No, the codes have not been cleared manually to see when they return. (The SEL stays on from Ignition through trip).I have a new tank of Premium fuel and the car runs so smoothly that it's hard to believe the engine light is on! I saw a great pair of videos by a mechanic/owner and learned that the slow start and lag in performance when you start out from a stopped position, or in first gear (automatic) a low rpm is directly related to the knock sensor in his case.The part is on order, as suggested, from Amazon. My mechanic has agreed to install it and do the diagnostic on the cat system, clear codes, to make the right call on the O2 sensors.Many Many thanks. I'll update what I find out. Link to comment Share on other sites More sharing options...
rae houghton Posted December 7, 2015 Share Posted December 7, 2015 this thread has me intrigued. Catalytic converters have been around for some time. Is it common for inspectors in all states to check for emissions efficiency? I have a CAT which I shall shortly remove from my parts car. I have been told I can get 40$ for it from one of the local recyclers. But perhaps it is worth keeping in case it should be needed. Of course in an above post by Subaru Fanatic...he suggests they live forever (or as long as the vehicle) so maybe keeping it is not necessary. Link to comment Share on other sites More sharing options...
rae houghton Posted December 7, 2015 Share Posted December 7, 2015 My parts car is a 1994 L- Series wagon. Link to comment Share on other sites More sharing options...
idosubaru Posted December 7, 2015 Share Posted December 7, 2015 not sure what your real question is. Emissions inspections varies by area - just depends where you live. Some places have no emissions - and you can drive around with a CEL for the life of the car, it's not part of any inspection at all. Some places you just can't have the CEL on. Some areas you can't have the CEL on and they do a sniff test - actually test the emissions out the tail pipe. yes you can remove the converter just fine without issues on any 2004 and earlier subaru. if it's equipped with a rear O2 sensor you'll get a check engine light. you can ignore it or try to trick it with a resistor/circult. it's benign either way. in 2005+ the rear O2 sensors are more than triggers and start to work into the ECU logic more, though i'm not sure how much. saving - that's up to you and wether it's a problem or not depends on your local laws or possibilities of moving to a place that has them. Link to comment Share on other sites More sharing options...
rae houghton Posted December 7, 2015 Share Posted December 7, 2015 thanks for your thoughts.... Link to comment Share on other sites More sharing options...
djmark7 Posted December 7, 2015 Share Posted December 7, 2015 in jersey they plug into the OBD plug and check the gas cap 2 yr inspection, unless new car then i think its a 4 yr inspection Link to comment Share on other sites More sharing options...
rae houghton Posted December 7, 2015 Share Posted December 7, 2015 Must be a big demand for converters. I just got a phone call from a guy representing one of those firms that collects them. He will actually come to my house to collect it...as they go up and down the coast to wreckers to collect them. I was offered 40$ by one company...don't know about this one...says it depends. interesting to know. Link to comment Share on other sites More sharing options...
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