P0420 Ford - 2004 f150 5.4

Question

plugs and coil packs have been replaced and when I accelerate over 40mph in low rpm's the truck shudders.

Answer 1

P0420 Catalytic effiecieny below threshold (bank 1)

When the engine can not fully contribute the catalytic convertor can not work properly. Misfire with this type of engine does not have to code up unless it reaches a max value. It common for the sparkplug boot to tear and create a lean misfire without setting a misfire code but setting a catalytic code condition. Also the fuel injector connector tends to get disconnected and it does not have to set a code condition.