I just got the spark plugs changed because my 2009 6 cyl. Jeep Wrangler (with 70,000 mi) was throwing code for a misfire. After the change, it doesn't run as smoothly as before. It chugs and skips, but does not stall. There are no warning codes showing up currently.

Is this considered normal for a vehicle with newly changed spark plugs? If so, how long should I expect it to take until it runs smoothly again?

  • 2
    If you did them yourself check that the correct wire is going to the correct plug aka the firing order is correct. If you paid someone bring it back and complain.
    – mikes
    Commented Apr 6, 2013 at 20:51
  • Thank you for your advice. I'll be taking it back next week. Commented Apr 6, 2013 at 20:56
  • 2
    Hard to believe some mechanics are so unprofessional as to return a vehicle to you in that state (worse state than when you brought it in)... Commented Apr 6, 2013 at 21:32

2 Answers 2


Your engine should run smoothly the second you change your spark plugs, smoother than before even, that's the whole point of replacing them. Otherwise, something else is wrong or was performed improperly.


Sounds to me like one of the spark plug wires isn't fully seated. I would check both ends of each cable and make sure they're fully seated.

The engine code it produced should give you some idea where to start:

  • P0300 - Random or multiple misfires
  • P0301 - Misfire on cylinder 1
  • P0302 - Misfire on cylinder 2
  • P0303 - Misfire on cylinder 3
  • P0304 - Misfire on cylinder 4
  • P0305 - Misfire on cylinder 5
  • P0306 - Misfire on cylinder 6

If the code is P0300, it could be many things, but usually indicates a compound problem such as improper firing order or multiple unseated plug wires among others. If the error is cylinder specific, check that spark plug (the plug wires are usually numbered if you don't know which is which).

Facing the engine from the front:

1, 3, 5, are on the left, front to back.

2, 4, 6, are on the right, front to back.

If you find one that's improperly seated, check them all, just to be safe.

  • This is a nice answer. I'm shocked at the number of votes. I edited the original question to shoot this up into the active questions list. Commented Apr 23, 2016 at 22:00

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