JohnCTT Posted September 13, 2019 Share Posted September 13, 2019 I'm getting a P0420 on a 2009 Legacy 2.5L (no turbo). This came after an engine rebuild required after a ringland failure of #4. Also, after the engine rebuild, the lower cat was found to be plugged with what I assume are parts of the manifold cat, so I suspect the manifold cat was damaged when the car was run with the dead cylinder sending unburned fuel to the cat. Changing the lower/back cat restored power and the car runs fine except for the P0420 code. Scouring this forum, I found many references to checking fuel trims before condemning a cat for a P0420. According to a cheapo scanner, the long term fuel trim is -7.7%. Is this within range for normal fuel trim? Thanks for any input. John Quote Link to comment Share on other sites More sharing options...
GeneralDisorder Posted September 13, 2019 Share Posted September 13, 2019 Pretty normal. Depends on when you took the reading. A high reading MAF at idle is indicative of it being dirty. Negative idle trims and positive cruise trims are typical of dirty MAF and/or dirty air filter. GD 1 Quote Link to comment Share on other sites More sharing options...
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