Random Misfire (P0300) and o2 sensor codes!!

[LongrangeLauber]

I will add this, since I've installed the new OEM y-pipe assembly, I've driven approximately 300 miles. My miles per gallon have gone up about 1- 1.5 mpg since then. Even lifted on 35s with 4.56 gears, I'm now averaging about 15.5-16mpg around town (slightly lower on the highway.) Yes, the speedometer is corrected for the gears and tires. I dialed it in perfectly based on GPS speeds and GPS distance driven. I know the computer is a little exaggerated with the mpg claims, and based on previous calculations it reads approximately 0.5 mpg better than actual. So realistically I'm getting about 15-15.5mpg. Still better than 13.5-14 mpg (hand calculated) when I had the 420/ 430 codes logged in the computer.

 

Need to take it into a shop and get this emissions test done finally 

[Jsdirt]

Nice! Guess that Magnaflow had some flow issues is what it sounds like. Good deal!  With fuel prices the way they are, it should cover the cost of the Y-pipe in about 90 days. 

 

That's impressive mileage! I'm on friggin stock 20" wheels, Corsa cat back, K&N cold air intake, and a Hypertech canned tune, and I haven't seen 15 unloaded in years. Towing I've been 10-11. Abysmal! I've got a Truck Norris cam and a bunch of other goodies sitting here waiting for my Magnuson supercharger to get here - saying it should ship end of June (Ordered mid March! ). Hopefully that gets me better mileage towing, since I won't have to keep it close to the rug for long periods of time. We shall see. 

 

I'll probably just start towing at 80 and send that all out the window. 

[WIElk]

Just thought I would add to this thread. 2018 Silverado 5.3. Had random misfire codes.  Replaced O2 sensor with NTK branded one. Problem persisted and actually got worse ( hind site is 20/20 here).  After unnecessary swapping parts found this thread. Removed new NTK sensor and replaced with OEM. Problems resolved.