GMC Troubleshooting Questions? Ask a Mechanic for Answers
Did you clear the codes to see which ones would return?
Did the engine feel like it was misfiring, or was it just a general loss of power?
With the codes you have, p0171 and P0174 are lean codes and P0300 is a misfire code. Sounds like the engine is starving for fuel. If you have not replaced the fuel filter in a while, I'd start there. Beyond that, testing fuel pressure will reveal whether the fuel pump needs to be replaced. You should have 60-66 psi fuel pressure with the key on, engine off. If the fuel pressure is lower than specified, the fuel pump should be replaced.