DupermanDave Posted June 2, 2009 Share Posted June 2, 2009 We had the check engine light come on. We had it scanned and the code was "Catalyst system below threshold bank 1" Possible causes are a rich A/F mixture, catalytic converter failure, or ignition system failure. The engine has also started smoking out the exhaust. Not a lot, just under high revving of the engine. Still drives fine. It's a 1996 subaru outback legacy. Body has 219k miles (im guessing original cat and O2 sensor) and engine has about 90k miles. Anyone know what could be causing the smoke and engine light? Im thinking O2 sensor, seeing as how it started smoking and suddenly started throwing the code. Quote Link to comment Share on other sites More sharing options...
porcupine73 Posted June 2, 2009 Share Posted June 2, 2009 That description sounds like P0420. Another cause can be exhaust leaks. The smokyness can be burning oil (how's your PCV valve? it might be all plugged up), or could be other smokes. Quote Link to comment Share on other sites More sharing options...
DupermanDave Posted June 2, 2009 Author Share Posted June 2, 2009 It's definitely burning oil. I recognize the smell. It's also leaking oil a lot more (I have another oil leak thread. May be the oils separator plate). I'm going to clean the PCV valve until I can buy a replacement and see if that helps. Maybe the valve is clogged up and causing more pressure to push the oil out causing it to leak more? (just a guess) Quote Link to comment Share on other sites More sharing options...
DupermanDave Posted June 30, 2009 Author Share Posted June 30, 2009 replaced the PCV valve and no change. What usually causes this? It's a California model with 2 cats and 2 O2 sensors. Is this generally a sensor issue or a catalytic converter issue? I was also told it could be an ignition system failure with sparkplugs and bad wires. I thought the catalytic converter material was supposed to break down over time, not clog up? Quote Link to comment Share on other sites More sharing options...
OB99W Posted June 30, 2009 Share Posted June 30, 2009 What usually causes this? It's a California model with 2 cats and 2 O2 sensors. Is this generally a sensor issue or a catalytic converter issue? I was also told it could be an ignition system failure with sparkplugs and bad wires. I thought the catalytic converter material was supposed to break down over time, not clog up? Burned oil can contaminate O2 sensors and/or the catalyst. A too-rich mixture, which could have several different causes, can prevent the converters from doing their job. Following are some links to P0420 info, most not Subaru-specific but interesting anyway: http://www.motor.com/magazine/pdfs/102006_09.pdf http://www.wellsmfgcorp.com/pdf/counterp_v7_i3_2003.pdf http://www.wellsmfgcorp.com/pdf/counterp_v7_i4_2003.pdf http://www.catalyticconverter.org/news/news_page.cfm?Key=codes-po420-po421-po430-po431&News=120 Quote Link to comment Share on other sites More sharing options...
DupermanDave Posted July 1, 2009 Author Share Posted July 1, 2009 So basically I need to find the root cause of the error code. Find what's causing the sensor or converter to fail. Where should I start? Spark plugs and wires is where I was thinking (keep it simple for now) and try and torubleshoot any misfires (if any). What else should I keep in mind? What else should I look into replacing? By the way, the smoking hasn't happened again. It went away after that first day it happened. Could not find the cause of that problem, but I'm still keeping an eye on it. Quote Link to comment Share on other sites More sharing options...
Mugs Posted July 1, 2009 Share Posted July 1, 2009 I run into this alot at the shop. Start by replacing the rear O2 snesor. If it is not working properly then it thinks the cat is below threshold and will set the 0420 code. I reccomend a Bosch or factory O2 not a cheezy run of the mill money saver kind. Hope this helps, and keep us posted. Quote Link to comment Share on other sites More sharing options...
OB99W Posted July 1, 2009 Share Posted July 1, 2009 So basically I need to find the root cause of the error code. Find what's causing the sensor or converter to fail. Where should I start? Spark plugs and wires is where I was thinking (keep it simple for now) and try and torubleshoot any misfires (if any). What else should I keep in mind? What else should I look into replacing? If the plugs and ignition wires haven't been replaced in a while, it wouldn't hurt to change them. Experience shows that using NGK plugs and Subaru OEM wires tends to be the most reliable way to go. If the O2 sensors are really originals, at 219k miles they're probably a little tired. If you have access to some test gear, monitoring the real-time operation of the O2 sensors and some other data could help narrow the diagnosis. If you have no way of looking at OBD-II data (other than reading trouble codes), I'd start with changing the front O2 sensor, which has primary effect on how the ECU sets mixture once the engine warms sufficiently. It's important that an OEM sensor be used. If you decide to change the rear O2 sensor, it's less critical and a good quality direct replacement would probably be adequate. Quote Link to comment Share on other sites More sharing options...
Log1call Posted July 2, 2009 Share Posted July 2, 2009 Since it is saying secondary O2 sensor is out of range, but it is not saying the front sensor is out of range, I would conclude that the cat is faulty. If there was a missfire the front O2 sensor would also set a code as well... probably! Having said that, I would check what the oil burning is caused by because the repair might not work of the engine refouls the new cat when you put it on. I'd also check the tune and for air leaks in the hope that the faul is marginal and a cleaner burning mixture will clear the code. Quote Link to comment Share on other sites More sharing options...
OB99W Posted July 2, 2009 Share Posted July 2, 2009 The ECU's determination of catalyst efficiency is based on a comparison of output from the front and rear O2 sensors. Under normal operating conditions, the front sensor rapidly switches from somewhat over zero volts to somewhat under 1.0 volt and back again, while the rear sensor switches much less frequently, and over a narrower range of voltage. That's because when the catalyst is doing its job, oxygen in the exhaust is used by the catalyst. A sluggish front O2 sensor can cause the ratio of gases in the exhaust to change enough so that the catalyst can't operate as designed. In that case, the rear O2 sensor could see fluctuations in oxygen level that it shouldn't. The ECU can inaccurately interpret those variations as a failure of the catalyst. Even if we assume that the cat should be replaced, at a minimum the front O2 sensor should also be changed to provide the new cat with a better chance of survival. It therefore makes sense to try changing the sensor first, since it's a lot easier to replace and less expensive than a cat, even if the cat is eventually changed anyway. Of course, if an OBD-II scan tool is available, the best approach is to look at some real-time data from the O2 sensors, see what the long and short term fuel trim numbers are, etc. That info would minimize the chance of a wrong interpretation of the cause of the code. Quote Link to comment Share on other sites More sharing options...
Log1call Posted July 2, 2009 Share Posted July 2, 2009 The O2 sensors can be tested with a voltage gauge. An analoge gauge that reads millivolts is best. With the motor running the voltage should range either side of four-fifty millivolts and it should go from one end of it's range to the other several times a minute. All sensors can be tested for correct operation... which is always a good idea before rushing out and buying new parts. Quote Link to comment Share on other sites More sharing options...
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