I have a 2005 Ford Expedition, from my dad, which stutters and hesitates to accelerate. Sometimes it barely has a problem and sometimes it's bad. It has new spark plugs and spark plug coils which the mechanic said would fix the problem. He reset the codes but soon enough it had the same stuttering and the check engine light is back on.

My dad swears up and down that it's the catalytic converters and each new mechanic I talk to says there's no way its the catalytic converters.

Can engine stuttering be caused by a bad catalytic converter and how could I test it to be sure that it is or isn't?

The ODB II sensor gives the codes:

P0300 - Powertrain Random/Multiple Cylinder Misfire Detected
P0302 - Cylinder 2 Misfire detected
P0304 - Cylinder 4 Misfire detected
P0305 - Cylinder 5 Misfire detected
P0308 - Cylinder 8 Misfire detected
P0420 - Powertrain Catalyst System Efficiency Below Threshold (Bank 1)
P0430 - Powertrain Catalyst System Efficiency Below Threshold (Bank 2)


In case it's relevant, when it starts stuttering, the AC blows warm for a few seconds and then it goes away. From a cold start it seems to run perfect for maybe 10 miles or so, and then may stutter on an incline or an attempt to accelerate.

In addition to the spark plugs and coils, the AC compressor, condenser, evaporator, fan, and clutch have been recently replaced. I also just replaced the radiator which had a crack in the plastic.

  • Fix the misfire condition first. Cat will not cause hesitation or misfire. The cat needs replaced also due to the code. – Moab Jun 27 '20 at 17:32
  • I'd really love to fix the misfire condition except that the mechanics say to change the spark plugs and coils again. What of this article here? autovfix.com/…. – Altimus Prime Jun 28 '20 at 0:06

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