Sounds like a primary O2 problem. It didn't start till the O2 warmed up and began reporting. The bad MAF probably fouled up the sensor with extremely rich mixture. 

 

GD

Makes perfect sense. Then I'll order the O2 sensor and hopefully be done with it. Thanks again, Erik

This should give some direction and help in diagnosis

 

Thanks GD! I have driven about 175 miles since I replaced the primary O2 sensor. CEL has not returned and over 21mpg, It's running normal again. So on your advice it looks to be better. My final question is why do I get a PERMANENT P2097 stored? Can a dealer clear it? But fortunately no light on the dash is on so I have cruise and traction control again. Erik

Not sure if it's been mentioned as I didn't read it, but I have a Toy that had the same issue, turned out the knock sensor was out of spec.  I think testing a Sub knock sensor should give you 5v.  A quick change on the toy and it fixed the power and mileage right away.  Something to think about if it hasn't been mentioned.

Have you ruled out one of the most common sensors to fail in the Subaru............the knock sensor?  Symptoms of failure?...loss of gas mileage.

Hi everyone. This is my first post on this forum. I have similar issues with my Forester SOHC '10 and I came across this thread. I noticed that the thread was started a long time ago, but since they are relatively recent posts, I thought it might be ok to post my question here...

So my Forester has 152,000 miles, driven in Ontario, Canada. I recently got the P2097 code, which led me to replace the rear O2 sensor, the first sensor to be replaced on this car. Obviously the code came back, so I got a better reader with live data capabilities. I am attaching a CSV file that contains data from a ride I just did with the car. I have included data for the two sensors, the MAF and the MAP. It was 21F outside, but the engine was still warm when I started it.

My understanding is that the front (wideband) O2 sensor reads about 5% too lean (lambda=1.05), while the new (narrowband) rear sensor reads too rich (0.94V). The long fuel trim around 10% would indicate that the ECU is adding more fuel than normal. Based on a lean front sensor and a rich rear sensor, does it mean that there is definitely something wrong with the front sensor, unless my new rear Bosch sensor is defective? Could a band MAF cause this issue (I don't see how that would be possible)? Note that the spark plugs and the air filter need to be changed soon (although I also do not see how this could cause these particular values from the sensors).

 

Thanks for your help!

BDDataLog.csv

Edited February 7, 20206 yr by kreyszig

2010 Forester. 185k miles.

I had the P2097 code and another one that said too rich instead of too lean. Bad gas mileage, CEL, flashing cruise. I replaced the MAF and rear O2 sensor with no affect. Just replaced the front O2 sensor and it's been running great for a week or so!

Hi,   i have an 09 Subaru forrester 107k that has been a nightmare car.  It's been in and out of the shops for 7 months. money dropped but no fix. started with an engine knock after I bought it.  Okay, no worries. I bought a motor from a company in Pennsylvania and had it shipped to Texas. I guess I picked the wrong shade tree mechanic who started the job but couldn't finish or even come close.  he still wanted money though...  I then took it to a mechanic shop with the engine next to it.  they put it in and got it running but with codes. p2097.  we replaced the 02 sensor pre cat  with a new dealer sensor.  than replaced the MAF sensor twice.  twice I was told it was done and given a bill and time to pick up.  each time I asked if it was fixed and if the monitors were set.  "No, thats not our job," was the response i got. Both times i had to drive it 80 miles and check engine light popped back on.  took it to another mechanic and after 2 weeks was told that the exhaust manifold was installed with a bolt instead of a stud and it punctured the head as oil leaks out of the bolt hole when the bolt is removed from the exhaust manifold but we don't do this type of work- gl. I took it to someone to do this work and after 2 weeks was told to replace the 02 sensor post cat.  next move is to replace the head which I have not done as this thread say post cat 02 sensor wont cause this variation more than 3%... starting to get really expensive and I need my truck to work not to let wife go to the grocery store