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Misfire | Subaru Forester

🚗 2011 Subaru Forester 2.5L

  • Car came in misfiring on all 4 cylinders at idle.
  • Customer previously has already replaced the spark plugs, coils, checked compression, replaced the front O2 Sensor, and the MAF Sensor.



🩺 Diagnosis

First, I took my diagnostic scanner, went to Engine ➱ Live Data, and started the car.

Fuel trims:

  • STFT +-3%
  • LTFT 0%
    • Both are well within normal range (+-10%)


Checked O2 sensors live data at idle, WOT, and introducing an air leak (unplugging brake booster hose) to see how well they detect rich/lean mixtures.

  • Front O2
    • Fluctuated above and below reference voltage (2.2V at KOEO) properly.
    • Reacted to WOT by going very low (lean) then high (rich).
  • Rear O2
    • Stayed mostly at 0.9V (a clue, indicating a rich mixture).
    • With introduced air leak or WOT, went down to 0.1V, meaning that sensor should be ok.

Checked fuel pressure: 54psi. Normal.

What can cause Rear O2 sensor to detect rich, but Front O2 not detect anything wrong? Especially considering that the customer has replaced so many parts already. Hmm.

  • Decided to check EGR Valve
    • As soon as I took it off the engine, I noticed a piece of something (catalytic converter?) not allowing the EGR valve to fully close.

Stuck open EGR valve causing misfire. 2011 Subaru Forester.


After removing debris, cleaning the valve, and putting everything back together, the engine started to idle smoothly.


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