Intermittent P0420 code on long trips - Subaru Outback - Subaru Outback Forums
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post #1 of 3 (permalink) Old 09-14-2010, 11:48 AM Thread Starter
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Intermittent P0420 code on long trips

Hi everybody!

I have been searching through the threads and haven't been able to find any problems similar to mine.

I have a 98 Outback 5MT with 155K miles. The problem I am experiencing is a intermittent P0420 code which started about 5K miles ago. The code only comes on during long trips (over 350 miles) but never during my daily 40 mile commute. When the code is thrown the car idles well and there is no loss of power and my average fuel economy is around 21 mpg.

I have replaced both converters, center pipe and muffler at 140K due to leaks and bad rattles. The converters are bosal and the rest of the exhaust is walker. I did change the oxygen sensors with beck arnley universal sensors. I was kind of tapped out after replacing the exhaust and before I learned that scoobys prefer OE o2 sensors. I also did a major tune up at 145K (new plugs, pcv valve, wires and filters).

Is it possible that the universal sensors are causing the error code? And is 21 mpg average for my year?

Thanks for your help in advance.

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post #2 of 3 (permalink) Old 09-18-2010, 03:01 AM
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I'm currently dealing with the same CEL code P0420 issue on my 06 Outback. My dealer quoted me $1.3k to replace supposedly bad catalytic converter. But through my online research, Oxygen sensor, specially the front one (before Cat Coverter) could be the culprit in many cases. So I'm going to replace that myself first and see what it does, along with doing few other things that i got from other people's tips (cleaning/replacing PCV valve, changing air filter/spark plugs & wires, checking for gasket/vacuum leaks etc).

But interestingly, another wisdom I earned from my iPod Touch free app called "RepairPal" is that, "replacing one with genetic/universal CO2 sensor often cause a trouble, so stick with OEM parts". I don't think I can shell out $180 from a dealer, but I wouldn't want to try those $22~$30 one either. So I'm going to try happy medium, $50~$60 Bosch oxygen sensor (retail $180, interestingly).

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post #3 of 3 (permalink) Old 09-18-2010, 05:43 AM
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To the OP,

Could be either a front o2 sensor, PCV valve or an intake or exhaust leak.

Check the easy cheap stuff first. You don't mention doing anything since the tuneup 10k miles ago ... check the PCV valve, clean it, make sure it's not sticking [I had the same issue, long trips it would code... turned out the PCV was dirty enough to stick and allow enough extra air in to the system to code]

Check that there are no leaks in your relatively new exhaust, bolts can loosen up with heating cooling and vibration. The extra air from an exhaust [or intake] leak will give the same result ... the sensor is showing too much unburned o2.

When the sensor sees extra O2 in the exhaust stream, it's first response is to add extra fuel to increase the combustion of the o2. If it can't add enough to "make it go away", it assumes the cat must be defective. If the leak is in the intake manifold, it generally will be able to add enough fuel to "make it go away" but, generally the leak is on 1-2 cylinders and the ECU adds fuel to all so ... you'll have a couple running rich. If it's the exhaust, it can add fuel all day long but since the o2 is coming in after the cylinder, it doesn't burn and it makes the whole thing worse.

So, check for intake and exhaust leaks [if the car has a MAF, check the ducting from the MAF all the way to the cylinder]. PCV.

Good luck
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