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Discussion Starter · #1 ·
I have logged the P0304 code (cylinder #4 mis-fire) twice in the past two weeks, intermittently. Anyone have the top three reasons that this could be caused by? I changed the plugs at about 100 k miles, at 128 k now. Coil pack? Something else? How critical is this error typically?

Thanks ultimate Elantra brain-trust!
 

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Faulty spark plug
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer


How is it actually running?
 

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Fuel. If all seems well till the code crops up, try a fresh fill up with added Techron a couple of times and see if it clears.

Also might be a good idea to pull that plug and make sure it looks good.

In my experience, when a coil goes bad, it goes bad. In the old days when you had one big can coil running all cylinders they could fail when they got hot. Cool them down and they would work for a while. The newer coils seem to just quit.
 

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Discussion Starter · #6 ·
I thought about Techron. I have followed Hyundai's recommendation to always add a can to a full tank each oil change. I just recently changed the oil, about 800 miles ago and did the Techron thing. Can you over Techron the fuel system? Is it safe to load another bottle in there? I'll also take a look at the plug this weekend.
 

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I thought about Techron. I have followed Hyundai's recommendation to always add a can to a full tank each oil change. I just recently changed the oil, about 800 miles ago and did the Techron thing. Can you over Techron the fuel system? Is it safe to load another bottle in there? I'll also take a look at the plug this weekend.
Your car will be fine if you put another techron in, but since you have been using it already, it's likely the fuel injection is the culprit. So you may just be wasting your money troubleshooting this at that point.
 

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Only time Techron can cause a problem is if you seriously overload the mix ratio. That can drop the octane from what a chem engineer told me.

But the injectors will definitely be clean!!!
 
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