I have my 2003 GMC 3500 4x4 with the 6.0 engine set all summer and started it up and I got a check engine light on. I pulled the codes and this is what I got p0134, p0171 and p0300. the truck runs real rough now. what do these codes mean and why did they set from not using it? How do I correct this? last spring when I was plowing snow with it it ran super but not needing it in the summer I didn't use it.

 

P0134 - The PCM detects that the affected H02S(Heated 02 sensor) voltage parameter is between 350-550 mV for 60 seconds.
P0174 - Average long term fuel trim cell value is above 23% for 6 seconds (Fuel's lean)
P0300 - PCM is detecting a crankshaft rotation speed variation indicating a misfire

My guess is you've got bad gas. Pump some new stuff in it, add some dry gas and reset the codes. Keep on drivin'. If that don't clear it up, perhaps a clogged fuel filter?

 

Listen to NullHead, he knows what he's talkin about !!!!!!

 

I will add some Chevron Techron and a can of dry gas and see what I get., thanks