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Code Po420 '04 Outback

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Newbie & 1st time poster. I've searched the forum and have seen other threads about this code with other models (Legacy) but nothing for my model. I'm not sure if the advice for other models will apply to my car, so sorry in advance if this is all redundant.


I have a 2004 Outback wagon, 6 cyl. The check engine light came on, so I took it to AutoZone to read the code and got PO420, Bank 1 sensor. I've already had the car in twice for this. The 1st time, a local shop replaced the cat convertor and the O2 sensors less than a year ago. The code popped up again about a month ago and I took it in and the same shop replaced the rear O2 sensor saying that it was probably bad. Now the light is on again. I'm planning on taking the car to a Subaru dealer for a 2nd opinion. I'd appreciate any thoughts/advice on the types of questions I should ask the service people to get a better idea what I'm dealing with. Thanks in advance.

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My Diagnostic Trouble Code chart says that P0420 means "Catalyst efficiency below threshold (Bank 1)".


Computer error messages often need interpretation. It could perhaps be a bad connection to a sensor or maybe an engine tuning problem that causes an incorrect mixture of gases into the converter. In my experience, cat converters and O2 sensors don't go bad that often and yours have already been replaced.


Time to get a second opinion from someone who knows how to troubleshoot these cars. Some dealership technicians aren't that skilled -They will change out parts because the "black box" seemed to suggest it.


Some years back, my '96 Impreza began displaying P0133 – (Upstream O2 sensor slow response – Bank 1, Sensor 1). I could see nothing wrong with the car's performance, so I deleted the alarm as an experiment. The alarm showed up again a few more times at intervals of days or weeks, then disappeared forever. Never did have a problem with the O2 sensor or the cat con.


An inexpensive code reader that can delete alarms is a nice tool to own. It lets you experiment a little and be more informed when you look for repair services.

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