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1999 Forester 4EAT 125K.


Seen multiple threads on replacing O2 sensors and air flow meters, here and other forums. Have replaced both, and fuel trim remains at or above +25% short term and long term. Intake and exhaust systems intact and no faults noted. Recalls and campaigns all taken care of. O2 sensor voltage front and rear read 0.000V most of the time. MAP, air flow, timing, and other parameters close to comparable vehicles. Car drives good most of the time when trim is high. Starts to run bad at idle when trim slowly climbs to 25 or 30, then switches to 0.0% and stays at 0.0%. Occasionally will have performance issues while driving, but not very often.


Looking for any answers or thoughts, no matter how far reaching they may be...

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In my experience, I have never seen the exhaust fix this code. Don't feel too bad that you have new O2 sensors, those are like spark plugs to me, higher interval, should be replaced every 80-120K miles.


I have seen:

Fuel Pump/Pressure Problem

Dirty Air Flow Meter

Disconnected air box.

2 Air filters installed on vehicle. Only the small box to the passenger side is to have one. Not the one behind the throttle body. Spacer belongs here. If you have an air filter that is cut to allow air thru this gap get the dealer part ($55-70) last I checked. Yes I have seen cars where people tried to make do with an air filter rubber seal instead.

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I had thought about fuel pressure.... noticed I could change trim values for the better by unplugging pressure regulator. Will inspect filter as well as fuel pressure tomorrow. Hope to have an answer by tomorrow afternoon.


Thanks Shawn!



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