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Old 11-26-2014, 05:23 PM
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Big "J" Big "J" is offline
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Join Date: Feb 2008
Location: Kaufman TX (East of DFW)
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Might be in left field but, what about the gas?

My 4.8 had the P0420 (Post CAT bank 1) SES recently and had to clear it a few times with my scanner. Thought my O2 sensor was going bad as well but, wasn't sure what changed all of the sudden.
Like you, it was running good and no similar issues. The only thing that was a common factor and kinda made sense, was the gas.
Oil companies went from summer blend to winter blend which, I believe contains more ethanol than the summer blend and will jack with our sensors.
So, to help narrow down possibilities I put a half bottle of seafoam in the tank and the other half used for an injector cleaning thru an intake vacuum line (brake booster hose). No more SES, better mileage and that V8 has new life at 182K.

Good Luck!
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2004 X5 4.8is Imola Red-228K ZF-GA6HP26Z-Sold
2006 X3 3.0i Mystic Blue-175K, ///M pkg 6-Spd Manual- Trading in 6/10/17 for a Mini too!
2003 Silver X5 3.0- sold
2000 VW Beetle 1.8 turbo-165K- Totaled 9/11/16
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