Check Engine Codes : P0300, P0420, P0171

The P0300 is there sometimes, not there other times, I have all new plugs and wires on this car. My ignition module went out on the car a couple months ago and I had that replaced, the people that replaced it said I might have a bad coil pack but its hard to know until the ingintion module was fixed. So I had them fix it incase I had other problems with the car. I would have done it my self if I didnt have to get to work the same day. Sometimes I notice a misfire other times I dont, sometimes if I clear the code it takes awhile before it will come back. I took the sparkplug wires off each coil back and measured at 200 Ohms and got a reading of 6.1, 6.1, and 5.8 the 5.8 worries me just a bit because its different then the other 2 but I am not sure if thats enough to make problems or not

The P0171 has been there since I replaced the head gasket, which took awhile because I could not fit the car in the garage and it was raining a lot. And once back together it was misfiring on 2 and 3 after some injector cleaner, cylender 3 stopped misfiring (at least not enough to throw a check engine light over) cylender 2 didnt come back after 4 tanks of injector cleaner so I replaced it. I think I still need to replace injector 3

As far as the P0171 I sprayed everything with carb cleaner, spark plugs, all the new gaskets, the throtle body and there was no change at all to the idle. Anyone else have any ideas?

As for the P0420 I think I want to solve the other 2 first since the P0420 was the last code that I had come up. And from what I have been told and what I have read is some of these codes could be causing the others.

The car does run fine other then a hicup every now and then. And if I start the car and run to the store, when I come out it dont like to immediately turn over like it does when its cold.

Anyone with any ideas?

420 is normally a plugged cat. if you had spark issues or coolant contamination can plug the cat or mess up the sensors. i would go look at the cat after a drive normally cats that are really plugged turn red or get unusually hot. plugged cat can also cause misfire and other codes

That cat didn't look red but I do sometimes smell like something is burning. But maybe it'* clogged just enough to cause problems. Sometimes the most fire disapears. Maybe I go have it checked.

not sure what that means but some one who knows exhaust well will know by how it looks and sometimes how it sounds when you tap on it

The p0300 and the P0171 could be related causing the P0420 to exist. Let the engine misfire go long enough will damage the convertor. The 171 needs addressed first, as the system is running lean and possibly causing the misfire 300 code. Since you replaced the head gasket, Id look at the vacuum lines, making the sure the oil dipstick tube is fully seat, and the intake gaskets are installed correctly. Sometimes spraying the intake is not enough to find a leak just by listening to rpm change. A scanner that reads fuel trims is needed while spraying around the intake and watching your data.
Another possibility and I have heard that after replacing the headgasket, sometimes coolant gets down into the exhaust pipe and will damage the 02 sensor causing it to skew on its readings.