P0449 2006 GMC SIERRA -

Question

My check engine light remains on. For a period of about 4 days, it would come on and go off, randomly, and even blink when I was driving. What is the solution route to repair this? I'm reading on-line that I need to replace my evap control vent valve, which appears to be located on/near the fuel tank. I have also read that GM has produced a new unit, due to faulty old unit, and it requires that I install an additional vent hose towards the front of the vehicle. What would be the initial, most cost effective steps to take in determining the problem and solution? Thanks.