My 06 SC Lariat 4x4 with 5.4 is making my crazy. After the converter locks, if I ease the gas pedal down I get a big "thud" like a dead miss. It only happens when the speeds is below say 75 and the rpms are below 2000. Kind of like if it is right at the point where the converter would unlock. I thought it was trying to unlock but slammed right back to the lock position. Went to trans shop to check it out. He came back with code P0430 (cat converter effiency below standard bank 2) If the trucks rpm's are above 2000 it does not happen. If you run it hard through the gears, no miss/thud. If cruise is on 6o mph and I come up to an incline, as the throttle start to increase the miss/thud starts. Any ideas at all? Truck has 123k trouble free miles.

 

Have you ever changed the spark plugs? Those are the first symptoms of a misfire.

You need to get that cat issue taken care of - it may be partially clogged which does that engine no good at all.

 

Thanks, glc. I will make plans for plug change Friday. What is the best route for cat fix? Replace with stock or aftermarket? Like I said this is very first bobble with truck.

 

Be VERY aware of the spark plug changing issues.

Aftermarket cats are a LOT cheaper than factory cats.