An Odd Case of EVAP codes

What are the symptoms of this issue?

P0449 Vent Control Circuit/Vent Control
P0446 Evaporative Emission Control System Vent Valve/Solenoid Circuit
I forget about possibly a 3rd code which was on this 2004 s10 V6 4.3L little truck. It had to do with Vent control high and/or low input. This typically means that the PCM is no longer seeing a value out of a sensor that it expects, or it can mean that there is a wiring issue.

It's not often with EVAP systems that there is a driveability issue or performance issue. Most often, it's just a pesky system which needs diagnosing so that you can pass emissions. The goal of the EVAP system is to capture fuel vapors and prevent them from going into the atmosphere. Hydrocarbons are actually very bad for air quality and this with other systems have managed air quality to be much better than what I remember them to be when I was a child in the 80's.

While driveability concerns or engine performance aren't typical with these when the purge valve opens without vent control, it could make an engine run rough - it did when I commanded at a lowest percent of 10 percent by a bi-directional tool for the purge valve to open. This allows the vapors to be burned in the engine. Typically, the purge valve has to be commanded at 30% or higher to effect idle, not just 10%.

What are the step by step testing suggestions? 

Remember, the same EVAP codes can head in many directions. I included typical directions from other techs, but it wasn't the case here.

P0446 Can be the vent being commanded open, but being stopped up or failing to open.
1. Be sure to check for the vent solenoid for opening and closing when commanded by a scan tool. This confirms that the truck's PCM has control over the solenoid. I usually use a large hose which fits over the vent solenoid while I breath into it. You should easily be able to breath out into it until it's commanded and successfully turns on.
2. Check the canister for flow restriction. (I skipped testing the canister for flow restrictions as I already saw the PCM had no control of the vent solenoid) In this case, the vent worked fine directly controlled with power and ground, but there was no ground and no control from the PCM.
In this case the Vent and Fuel Tank Pressure (FTP) sensor both were missing ground. To see this, the gas tank had to be removed. The FTP was showing in live data at 13.5 mm/Hg whether it was plugged in or not.
3. When unplugged sensors read the same this is a hint wiring is bad. Inspect the wiring harness up the frame. Found damage and repaired. If not so obvious use a wire sounding tool to test your way up the harness to find the break. Generally, Chevy wiring harnesses are too thick for my sounding tool to accurately hunt for the real breaks in the wire. With many wires in the harness it blocks the "sound" and your tool indicates the wire is broken where it is not. Maybe PowerProbe's wire sounding which came from the Power Probe 3, while expensive isn't the best for the job. Otherwise known as the ECT2000.
It is possible the PCM is not providing ground as both of these circuits are directly grounded by the PCM. The vent solenoid is ground side switched.

What fixed it?

Wires were repaired.