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Hello, I am new to forums, but in desperate need of help! 

I have a 2012 Outback with the 2.5.  On fathers day it started having trouble.  Hitting the gas pedal there would be a long delay of no acceleration, an occasional misfire, then the rpms would come up & the car would take off.  My code scanner showed a P0172 (rich mixture bank 1).  

I cleaned the Mass AirFlow sensor, replaced the Air Filter & threw a bottle of Techron in for good measure.  The problem had no change.  I talked to my local dealer who advised it was probably the air fuel sensor gone bad.  I purchased a new one (from them at the cost of 200 dollars) and installed.  The problem still persisted, however with the sensor unplugged it drove normal.   I could'nt figure it out, and didn't want to go broke buying everything that could cause a rich condition.  I made an appointment and dropped the car off at the Subaru dealer. 

The dealer advised they couldn't replicate, but changed my airbag (recall).  I went back the next day and drove it 15 to 20 minutes and it did it again.  They diagnosed a bad MAF sensor and replaced.  I picked the car up and asked if the MAF sensor was bad why didn't it trip the code?  I was told it doesn't always trip a code but thats why our scanners are 10 grand, and far more sophisticated then the 50 dollar scanners.  I paid and left the dealership.   I found the fuel light was now on & the car had over 90 miles put on her (I dropped it off with a little over half a tank  of gas and never drop my fuel below 1/4 tank)!  I didn't make it 2 miles from the dealer and same symptoms - same problem, no acceleration, misfiring.  I took it back to the dealer, they reconnected their scan tool and told me the car was fine, there is just no gas in it.  All codes are reading normal.  The dealership said we put a quarter of a tank of gas in and its fine!  I drove home with no issues (about 5 miles).  

Later that day I took the car back out and after 15 minutes same trouble!  Made it to the gas station to fill up, and the barley made it to the dealer. The car even stalled! I again had to leave it.  

Trouble light appeared, same P0172 code.  Dealer said they unplugged the main harness and reconnected and the car is now fine.  I asked if the connectors were corroded and was told not really.

This car carries my wife & two very young kids (in car seats) and I have some real safety concerns with this car & the dealers ability to diagnose and fix.  As it has been almost 3 weeks without a car!  They've already told me, we can't duplicate, or it's fixed, and it wasn't.   This is very frustrating especially since the car was purchased new, well maintained, and only has 28k miles!  

Any help or direction would be greatly appreciated!  Thank you!

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Sounds like you need to monitor the AFR sensor output while watching fuel trims. Being intermittent and the sensor being new I would say you probably either have a wiring issue (perhaps with that main harness plug) or an ECM problem. It's going to take driving and monitoring and till the problem shows up and you need someone watching live data that can interpret it correctly and potentially do some circuit probing and oscilloscope work to narrow the culprit. 

GD

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Dealership said they unplugged & plugged the main harness in & that fixed it.  I asked if the harness was corroded and they said no.  I asked them to call an engineer and have them review the data.  Can a simple plug & unplug fix the car, or is there something deeper going on?  

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