Had a P0455 (newer version of P0440) on my ’04 Grand AM with the Ecotec engine. For GM 4 cylinders (but not GM 6 cylinders) GM did not program in the DTC test where the PCM will test the circuit condition of the purge valve solenoid and vent valve solenoid so with the key on there will be +12 v on one wire. The other wire is PCM grounded so do not mess with it. If you have a shop scanner that can start the EVAP test and can feel the solenoids working. If the purge solenoid turns on and off with a pair of jumper clips then test then remove the purge solenoid from the car and use a 1 ft length of (IIRC) 1/4″ fuel line on the intake port of the purge valve and suck on it. There should be no vacuum leak and when jumpered it should be like sucking through an open hose. Do the same on the purge port side and you should have the same results. If not then the valve is bad. Test the vent solenold the same way as you did the purge solenoid by checking voltage, applying voltage and applying vacuum. Note that the only reason for the vent valve solenoid is basically to test the EVAP system but there is not test to test for it closing completely and not leaking enough vacuum tripping a P0440/P0455. From what I have seen online it is the most common component to fail and cause a P0440/P0455 on the GM car or truck. Stuck closed/dead purge solenoid will do the as well as a rubber hose that is bad and the fuel filler neck are quite bad for failing with a pin hole leak on FWD GM cars. Fuel caps that leak are quite common and can cause a P0440/P0455 but usually in combination with another leak. Check all visible plastic Evap lines in the system carefully, double check all rubber hoses, and triple check any rusty looking steel components in the system (including the fuel tank where possible).
FYI I changed out the fuel cap first (no luck), the purge valve solenoid (leaky when closed which would cause a P1441/P0496) and the vent valve solenoid (actually held vacuum off the car but very dusty inside with a spider nest ) using these testing techniques. Not the best way to do it for sure, but many mechanics are clueless on how to check these systems with smoke machines and at $55 from Rockauto.com for the parts much cheaper then taking it to a mechanic.