P0300 Story, 2000-5.3-173,000

This for those that have ever had a P0300 error code....this is my story:

So a week ago my 2000 Silverado, 5.3, with 173,000 starts misfiring and the SES light comes on. I take it into Autozone to have the code pulled...and of all codes its a P0300, random misfire across all cylinders. So before I leave Autozone, I purchase a new PCV valve (change every 12mo), a new fuel filter (change every 12 mo), and some MAF sensor cleaner.

So I get home and clean the MAF sensor and change out the PCV valve...and the error code goes away. The next day when I run the truck in the highway again, more misfiring and SES light on again. Goodgrief...so I call my mechanic and ask him for a little advice and he says the P0300 is not an easy code to diagnose without looking into somethings.

Needless to say, I took my truck to him and let him do his thing over 3 days (he takes his sweet time, buts gets it right the 1st time). Here is what he came up with:

He removed all 8 injectors and hand cleaned them...said they were caked with crap and cleaned the fuel rail lines; he asked me where I was buying my gas and I told him Walmart...and he said..."I can tell". That caught me off guard...anyway with injectors and fuel lines clean and back installed...the misfiring continued. So he went to the exhaust...pulled the Y pipe (includes 2 CATS) off and started the truck....and miraculously the misfiring stops. So he bolts the left side of the Y pipe back up to the left manifold and no misfire... so then he removes the Y pipe from the left side and bolts the right Y pipe side to the right manifold and the misfiring come back.

So now we know its either the O2 sensors or the left CAT...and we already knew the O2 sensors on the Left side of the Y pipe were good...so we switch them to the right side to see if the misfiring stopped....well it didn't...so that meant the O2 sensors on the left bank were still good but the Left CAT had crapped out. We cut the left CAT out and put a light in it....and there was no light coming out the other side...completely clogged up. He slapped a new CAT on the left side and all misfiring went away and truck runs like a champ now.

Well, back to the walmart fuel...he said from experience walmart gas is bottom of the barrel and has noticed in the cars he has worked on that had walmart gas...the fuel contained alot of water...as mine did. Also...don't ever let your fuel gauge run below a Qtr of a tank...all that crap that settles out of the gas in your just gets pumped to you fuel filter and then to your injectors...crapping them up. My mechanic said my fuel filter was also completely clogged...he cut it open...and it looked like it had been laying in the sand for year.

All that and it set me back $513.00...new CAT, labor, and diagnostic. NOT BAD!

you are very fortunate to have a repair person that takes the time to properly diagnose these problems ....and this problem with most shops would just start replacing parts until they got it right at your expense...


normally if the cat converter is suspected of being plugged removal of the upstream O2 sensor and leaving the hole open will reduce the pressure to stop the loss of power/miss fires..


swaping O2 sensors I recommend as an aid to troubleshooting as the sensors are expensive...


I would add a few onces of marvel mystrey oil to the fuel for a few tanks,,this will clean out the fuel system,,,after that I would replace the fuel filter and as your mechanic did cut open to see if debris has reduced..

I'm sure your truck has plenty of power now..