Now to figure out this missing problem

[99Yukon4x4]

My '99 Yukon had the same misfire issues. Once the check engine light came on, I had it scanned and found my #4 plug was bad (after I had replaced them 7 months before). It also had oil deposits on it. I normally pull the plug once a month and clean it up, and it runs fine.

NOTE- Body had 191K on it, but the engine is not original. Not sure what year, or what it came from. Other than that issue, drove it from Tampa Florida to Philadelphia loaded, and handled fine, and still got 17mpg on it.

 

[97blueyuk]

changed the o2 sensor, cleared the codes, the 0300 came back a few mins later. let it run a lil bit (but did not drive) and the cel went out. checked the codes again and nothing. still runs a lil rough though, i will take it for a ride later and see how it goes.

thanks for everyones help