2003 GMC 3500 6.0 Engine Fault Codes P0134, P0171, and P0300: Causes and Fixes
My 2003 GMC 3500 4x4 with the 6.0L engine has been stored all summer and I recently started it up. The check engine light came on, and the OBD-II scanner revealed three fault codes: P0134, P0171, and P0300. The truck now runs very roughly. What do these codes mean? Why did they trigger after prolonged non-use? How can I resolve this issue? Last spring, when I was using it for snow plowing, the engine ran smoothly—no issues at all. But during summer storage, I didn't use it regularly.
Begin by checking if the oxygen sensor (O2) is stuck in a biased state and whether it can properly switch between lean and rich conditions. A faulty O2 sensor could be causing all three codes, but it might also stem from another underlying issue unrelated to the sensor. Additionally, inspect for vacuum leaks or low fuel pressure—both are common culprits in 6.0L diesel engines. Also, consider whether the tank contains old gasoline that may have degraded over time.