P0303 code '03 Yukon

[Bedhead514]

I've been getting a P0303 code on my 2003 Yukon (5.3L) for a few weeks now. The truck has approx. 148K miles.

Initially I thought it was the coil pack (spark plug and wire looked fine) so I swapped it out. The check engine light went off almost immediately and the car ran fine for about 48 hrs, then check engine light came back on with the P0303 code. After talking with a mechanic friend, he said the coil pack may be defective and recommended I take it back. I swapped out the coil pack for a new one, and just like before the code was gone and light went off. The truck ran fine for over a week this time, till again the check engine light came on and the P0303 came back (about 3 days ago).

Yesterday afternoon I put a new AC Delco spark plug (GM rec) in cylinder 3 and changed out the wire, no change with code still there. After that, I swapped the coil pack and wire for cylinders 3 and 4 to see if the problem would switch to cylinder 4. Unfortunately it didn’t, I now get a P0300 code.

Any idea what’s going on?

 

[nikogto]

PO300 is a random misfire code, have you had your intake gaskets replaced yet? Does it ever run rough when first started? Could be O2 sensor, restricted cat convertor.

 

[Bedhead514]

I replaced the intake manifold gasket last fall, after replacing both knock sensors.

Out of curiosity, I swapped the coil packs back to their original sides a little while ago and the code went back to P0303. I'm not getting any other codes on the scanner I have.

 

[SLCHOE]

I replaced the intake manifold gasket last fall, after replacing both knock sensors.

Out of curiosity, I swapped the coil packs back to their original sides a little while ago and the code went back to P0303. I'm not getting any other codes on the scanner I have.

Since it sounds like you are clearing the code after every change you make, I would not discount the P0300 Random Misfire code. That heans more than one cylinder has missed at the same time.

Can you feel it missing? If so do a power balance test and pull the #3 plug while it is idling.

Do you have spark? Hold the spark plug wire boot close to a ground (block/alternator bracket/whatever is bolted to the engine and metal) and check for spark to jump.

If you have spark and there is no change in idle roughness then that cylinder has an issue. It may be fuel or compression.

 

[Bedhead514]

I think I got some good news yesterday.

I unhooked the battery in the morning and let it sit for a couple of hours, to clear the codes again. I drove it a little in the afternoon, check engine light came on as expected with engine still running rough.

I scanned it when I got home and pulled a P0304 code (no longer P0303 code). This means the problem has shifted from cylinder 3 to cylinder 4, and it's either a bad wire or another bad coil pack. I'm going to check for spark this afternoon and hopefully can figure out which one is bad.