2009 2.5 4EAT, Premium, 135000 miles. Ive been getting a P0420 for the last year. Even after a new exhaust was installed, including new sensors. Sadly, for me, I just reset the code each time it occurs as the car generally runs "OK". I drive about 70 miles each day (35 miles each way) for a commute that is broken up about 30% highway (65-70) and the rest back roads (45-55). The fuel economy has been anywhere from 22 to 26 fairly steady, but generally speaking it usually stays in the low end unless you really baby the gas pedal. Once in awhile I have had some bad misfiring on #4, tossing a P0304 code. A mech told me it was strange as even with new plugs, wires, etc. it was still occuring, then suddenly after swapping wires between 3&4, it cleared up. Recently this has come back in wet weather. I just put up with it as it tends to go away after a few miles. AS for the P0420, I have had a few mechs tell me that since I have a newer cat, it is most likely a bad sensor. Last weekend I replaced it.
Monday drive all was well. Tuesday, massive misfiring, it was raining like mad. Wednesday a bit better, Thursday fair running. This morning I refilled, and the morning commute was fine, averaging nearly 27 mpg. On the way home, it got weird. Suddenly I get the flashing CEL, AT TEMP, ABS, Traction Control, etc. I have seen these before, but this time the AT TEMP light flashes red then blue. I scanned it and came up with at first the P0420. I reset it and the lights remained. I scanned it again and came up with the P0600. ONce this was done I noticed something strange. The car was acting like it was in one gear, and the fuel economy was increasing. I drove about 10 miles this way, running in one gear at about 3000-4000 rpm. fuel econ at start was about 27, at stop it was over 38mpg (never have I seen it on average above 29). I stopped, and shut off the car for a few minutes, then restarted. all was clear. The car shifted fine and ran like normal. Fuel economy stayed high which was very odd for this car. AFter much start / stop / on / off the econ was around 28.5 when I shut if off. I really do not know what to make of this. I was going to look at the misfire issue again this weekend (check plugs, wires, coil, etc.). Oh, for those unfamiliar, the P0600 is a serial connection fault. I am still researching that.
Monday drive all was well. Tuesday, massive misfiring, it was raining like mad. Wednesday a bit better, Thursday fair running. This morning I refilled, and the morning commute was fine, averaging nearly 27 mpg. On the way home, it got weird. Suddenly I get the flashing CEL, AT TEMP, ABS, Traction Control, etc. I have seen these before, but this time the AT TEMP light flashes red then blue. I scanned it and came up with at first the P0420. I reset it and the lights remained. I scanned it again and came up with the P0600. ONce this was done I noticed something strange. The car was acting like it was in one gear, and the fuel economy was increasing. I drove about 10 miles this way, running in one gear at about 3000-4000 rpm. fuel econ at start was about 27, at stop it was over 38mpg (never have I seen it on average above 29). I stopped, and shut off the car for a few minutes, then restarted. all was clear. The car shifted fine and ran like normal. Fuel economy stayed high which was very odd for this car. AFter much start / stop / on / off the econ was around 28.5 when I shut if off. I really do not know what to make of this. I was going to look at the misfire issue again this weekend (check plugs, wires, coil, etc.). Oh, for those unfamiliar, the P0600 is a serial connection fault. I am still researching that.