P0411 Secondary Air Flow Fault - 2006 Pontiac Grand Prix (3.8L N/A) | O2 Sensor & Heater Resistance Issues

New member here — apologies for the long post, I'm trying to understand what's causing this recurring issue on my 2006 Pontiac Grand Prix with a 3.8L naturally aspirated engine. I've been experiencing intermittent P0411 (Secondary Air Flow) and occasional O2 heater resistance codes (P0053/P0054), even after replacing the secondary air flow check valve. The issue appears to be triggered specifically when starting the vehicle, especially in warmer weather — it's more frequent during summer months and seems to occur daily under those conditions. My friend has a GM-specific OBD-II scanner that can command system functions. During testing, he turned on the blower fan and secondary air valve while the engine was off. We observed clear airflow at the exhaust manifold. Then, after starting the engine, we toggled the O2 sensor heater — the sensor responded appropriately when activated, indicating the system is functional. The code typically appears only during initial startup, which makes sense because the secondary air flow system is tested at start-up. I've noticed that in colder weather (freezing to sub-freezing), the issue occurs once every 10 days or so — but it's much more common in warmer conditions. Using an Android OBD-II app, I access Mode 6 data. Most readings show TID$03 (normal), but occasionally I see TID$16, which indicates a failed secondary air flow test. I've checked all major ground connections — they appear tight and secure — especially around the secondary air system and O2 sensor locations. However, I'm wondering if there's a specific grounding point that should be prioritized for testing. The fault doesn't cause any drivability issues except for disabling remote start when the check engine light illuminates. As such, I clear the code regularly. On cold mornings or first starts of the day, the vehicle may stumble slightly during startup — very rarely, it stumbles twice before responding to a slight throttle input and then starts smoothly. I'm now considering whether faulty O2 sensors could be contributing. Occasionally, the system reports 'stuck open' or incorrect air flow readings, with one instance out of fifty showing an incorrect reading during a test. Could this point to failing O2 sensors? Are there any reliable methods to test O2 sensor health? Is it possible that the heater element in the O2 sensor is malfunctioning — especially since these heaters are critical for cold-start operation? Thanks for any insights or troubleshooting steps! P.S. I’ve seen similar reports on other 2006 Grand Prix owners with P0411, but no clear resolution yet.