I don't know what to make of this one. I pulled the engine out of my 2002, 2.5 liter Impreza Outback a few weeks ago to put a new clutch in. Best mileage I've gotten with this car previously is 27 MPG. A few days after getting it all back together, the "check engine" light came on with code PO136. Rear Ox sensor or circuit I assume? Seems odd that it happened just after having the engine out unless one of the electric harness plugs is not making perfect contact after having apart. I'm not even sure any of those two big plugs are IN the OX circuit.

 

I decided to drive it awhile to see what kind of adverse affects this potential problem has on fuel mileage. Well? Ran two tanks and got 30.5 MPG and 31 MPG. That's the best this car has ever gotten! Now I kind of hate to "fix" it, but I'll have to sometime to pass NY emissions inspection.

 

I don't know what the car's computer thinks the rear OX sensor is, or is not doing - but I kind of like it.

 

I had a OX sensor go bad in my 1979 Datsun 280ZX and when that happened, mileage dropped from 24 MPG down to 16 MPG and the tailpipe got covered with black soot.

 

On this Impreza, I can't imagine what could happen to make it jump up in mileage instead of down with this so-called malfunction.

 

 

 

That is odd.

I have replaced O2 sensors and gotten worse mileage with the new one in.

Id be tempted to take it in for inspection, as is, just to see what they come up with.....

i have a 99 outback with a 2.5 and when i bought the car with 100k miles on it, i was getting 25 mpg steady.about a month after i had the car the check engine light came on and the milage went to 31 mpg i went to the subaru dealer and ask if this would lean out the fuel mix to much and burn up my motor, but the head mechanic said that the computer would never let the mix that lean, and he suggested i leave it that way untill inspection, which i did ,and the light went out about 3 weeks later and the milage went back to 25 mpg. bummer! I would sure like to know how to make it do that again, that was in 2005 and i have 240k on the car now.