P0440 Fault Diagnosis - 2001 Toyota Tacoma (184K Miles) | Canister, Valve, and Ground Issues

I'm experiencing only the P0440 diagnostic code on my 2001 Toyota Tacoma with 184,000 miles. I've already replaced the gas cap and inspected all emission-related hoses and electrical connections to the evaporative system valves. When opening the fuel cap, there is no detectable pressure in the system. I disconnected the 12V connector from the evaporative valve and measured voltage across its terminals — reading approximately 4.3 volts. However, when measuring each terminal of the connector to chassis ground, one reads 12V and the other reads 0V. My question: Is the 12V supply always live (hot), with ground being switched? When does the 12V signal actually activate the valve — during ignition on or while the engine is running? I attempted to manually test the valves by blowing air through them and found that airflow works in one direction only, but not the reverse, even without applied voltage. This suggests a potential issue with the canister or its associated components. Is there a specific sensor dedicated to monitoring the fuel vapor canister? I've read other posts where users report multiple codes alongside P0440, but in my case, it's isolated — just this one code. How can I confirm whether the problem lies with the canister itself or another component?