Code P0446?

My 1996 Blazer 4.3, 4x4 has the service engine light on and I read a code P0446 with the OBDII Scan Tool, which is a Evaporative Emission Control System Vent Control Circuit Malfunction. I know sometimes this can be due to filling upyour tank while the engine is running. And I did not do this, does anyone else know what could be the cause of this? Or what steps i should take to correct it. Appreciate the help.

Thanks,
Jon