I have a 2006 PT Cruiser with a mid fire code on cylinder 4

If you turn ac on and it shakes it means your taking on extra load probably not the cause of the misfire. If they tried to clear the code with no success the it has to do with ignition or a bad ground in that circuit. check to see if there has been a recall on ignition switch. here is the link http://www.carcomplaints.com/Chrysler/PT_Cruiser/2006/engine/engine_light_on_and_idles_rough.shtml

Changing the fuel pump would not do anything to help or cause a misfire code. The fuel injectors need the fuel pressure to be 35-45 psi. to operate properly. The 1st thing is to make sure you are working on the correct cylinder. I was an ASE mastertech for close to 30 yrs. and I seen it happen before by other mechanics. If you clear the code and it pops right back you still have a issue with that cylinder. Make sure that the plugs are gaped to proper specs. To make sure you don't have a defective plugs switch #4 with #3 and start it up and see if misfire changes to #3. if it does then you know you have a defective plug. Do the same thing with your coil packs and the plug wires. If there is no change after switching each item(one item at a time)you'll need to check your connections and make sure you're getting power. Also make sure you don't have a bad vacumm leak. I would do compression test if compression is low you might have a bent valve or broken valve spring. BTW if you bought coil pack from advanced auto parts store you might have a defective (new) coil pack. It has happened to me before when I let my customer buy his own parts. Make sure you put anti-sieze on spark plug threads or you might not get the plugs out without stripping the threads on your next tune up. I hope that helps you out. Good Luck. Make sure your battery connections are good. Check all your ground connections because a bad ground could cause all kinds of issues.

The coil pack came from O'Reilly (I think that's the correct spelling. Its the 2nd one we've tried. Tried switching the plugs. Also used the grease for easy slip out on the plugs so they wouldn't strip. However when one of the battery terminals was changed. The light went off. Not to mention. The dashboard lights went crazy a few times back and forth than finally settled. Still at least 5 machines read cylinder 4.

Was it the negative battery terminal that you replaced? If it was, you might still have a problem with your ground. How old is your battery? Does your battery go dead if the car sits overnight? Does the battery terminal bolt onto the battery cable through an eyelet? Or did you cut the cable and put an aftermarket batt.Terminal? Also check your connections at the starter. Let me know so I can try to point you in the right direction.

You may need a camshaft sensor

I have the two codes for a 2006 pt cruiser po128 and po340

Pt cruisers have bad wiring and several issues with ground connections. Check all ground connections on the inner fender wells on both sides of the vehicle. I have a 2006 and the ground connection on passenger side fender well came off in my hand they were so badly corroded. I crimped a new ground wire and relocated the ground to a frame location and restore my lighting, fuel pump and horn to say the least. Also, check for vacuum leaks as this is a common reason for the 303 misfire code and rough running, This will also cause some silly shifting uphill or under acceleration. Using a propane torch canister to see where the idling improves near the manifold and cylinder locations and vacuum lines helps to find the leaks. I hope this helps.