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Subaru emission system error code puzzle

About little over a year ago, I bought a used 2005 Subaru outback that was rebuilt from salvage. It’s never had the check engine light off for long… soon after one faulty part is replaced, the light comes back on with a new code related to the emissions system. I’m suspicious there is an underlying problem that no one (of the five mechanics I’ve taken it to) have been able to figure out. Here is what has happened:

– January 2019: P0171 code (System Too Lean for Bank 1) – fixed by replacing front O2 sensor

– October 2019: P2096 (Post Catalyst Fuel Trim System Too Lean Bank 1) – fixed by replacing back O2 sensor

– September 2019: P0420 code comes on. I had to save up to replace the catalytic converter in March 2020.

– Now, April 2020: P0131 code (Oxygen Sensor Circuit Low Voltage Bank 1 Sensor 1).

My mechanic is now telling me to replace the front oxygen sensor again to fix this problem, just over a year since I first replaced it. He didn’t see any wiring issues or anything else wrong.

It seems like there has to be an underlying problem causing these parts to keep going bad. Any advice? I don’t want to just keep replacing these parts, especially since I just put in an expensive catalytic converter.

Thank you!


Do not make the oft repeated mistake of taking each code at face value and assigning a component to replace because of said code.

A P0420 for example does not dictate that you immediately run out and buy a new CAT… not by a long shot. Id suspect the cat long after other items such as O2 sensors and exhaust leaks and imho the cat is rarely ever replaced… only times if ever had to replace one was in situations of physical cat damage like being crushed or clogged solid with head gasket fixes in a bottle…etc

Codes are not part swapping guides, please dont make that mistake

Codes are a road map of items to look into and then to verify as good or bad using your experience, tools and mechanical flow charts. Codes also will bring into question the integrity of many “circuits” I see most people completely ignore the word “circuit” in the code description… a real big… real bad mistake.

People make these code / part swapping mistakes constantly. Be careful and take your time to verify components and or circuits where applicable… no assumptions and no relying too heavily on codes. There are ways to verify your existing components… dont overlook that…


Thanks for your advice. I’ve definitely been frustrated by mechanics not being able to find anything wrong and just recommending replacing the part. I was told that the cat went bad because I had driven for a bit with a bad front O2 sensor. Unfortunately I don’t have enough car knowledge or skills to inspect everything myself and have to rely on mechanics. This time I’m not going to rush into accepting the recommendation to replace the O2 sensor though, since it was just replaced I feel more confident that it’s something else.

If anyone has any specific components I should verify the mechanic has checked while troubleshooting this code, that would be greatly appreciated. So far I’ve read that possibilities are:

  • faulty wiring around the sensor
  • vacuum leaks
  • fuel pressure too high/low
  • leaks in exhaust system
  • other problem in the sensor circuit

Thank you!

Look at it thisa way… instead of changing out a cat you can do just the sensors. Its a bargain…

I would have replaced BOTH o2 sensors… and I would have done that at the same time. Then checked for exhaust leaks… and see whatchagots code wise… you prob wouldnt have any. That is how that should have gone.

Its hardly ever a good idea to replace one o2 sensor… you can if you are a mechanic and like to “play” and dont mind the learning experience… but im sure many here will agree… do both sensors… they are a team effort and rely on each other to keep that code off.

Your first problem is that the car was a rebuilt salvage car. (maybe a flood car??) Cheap cars are often not so cheap in the long run.

The second problem is your “mechanics” are incompetent. When I read your list of codes and repairs I kept thinking Nope!, Nope!, Would not have done that, waste of money! Ok, don’t replace another O2 sensor until you can see the failure on a scanner.

The 3rd problem is you keep going from mechanic to mechanic. 5 pros with no prior history on the car is not likely to get it fixed, as you have found.

If you are taking the car to Pep Boys or Midas or any other big national chains, stop that. Find a well-rated independent auto repair shop and establish a relationship. Be honest with them about the car’s history. Be prepared to pay a bit more. Cheap work is no good. Good work is not cheap! Overall, you will pay less.


+1 on all of Mustangman’s points!


And I will have to +2 for the veracity of Mr @Mustangman

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Yes you need a scan tool with live data normally to figure this stuff out. In this particular case, you have a circuit failure though. You will likely need to unplug the o2 and test the wiring harness connector for proper ground/voltage. If it checks out, cheap brand sensors apparently come back to bite people sometimes.

Your original issue could be many things. Are there any specific symptoms? Something was causing you to run lean originally. Again, you’d need a scan tool to go further…and if you got all that money for those parts, you can get you a cheap scan tool.