P0440 Fault on 2003 Pontiac Sunfire: Gas Cap or Evaporative System Issue?

I've been experiencing a persistent P0440 diagnostic code on my 2003 Pontiac Sunfire since June 2006. I visited the dealership three times, and each time the technician used a scanner to confirm the P0440 code. After replacing the gas cap and clearing the code, the check engine light returned—first after about 24 hours, then after 3 days, and finally after 6 days during my third visit in October. The first technician advised that the most common cause of P0440 is a faulty or improperly sealed gas cap. He also demonstrated how to close it correctly—which I already knew. After this third visit, I tried keeping my fuel tank as full as possible, assuming this would prevent the issue. However, the check engine light reappeared after just 6 days. I’ve noticed that during summer months, my tank was often below half-full, which may have contributed to the problem. Now I’m concerned: is it really a gas cap issue, or could there be another underlying component—such as a malfunctioning evaporative system solenoid? Given that this issue has persisted for over 10 years and no major repairs were needed in earlier model years, I'm hesitant to spend hundreds of dollars on what might not be a serious problem. I’m looking for reliable information about the actual root cause of P044 or whether it's simply a sensor-related false positive.