Hmm. If able, are you able to do at least one of these to cross-check?:... As I mentioned in my explanation, I tested with both resistor types (2.2kΩ and 4.7kΩ) and even completely removed the chip. In this configuration, when running the I2C scan, I should be getting an AcknowledgeCheckFailed(Unknown) error - not a Timeout. Here's the strange part: Using the exact same setup, when I simply: - Remove my manually placed breadboard resistors - Connect the pre-made DS1307 RTC module (which has built-in 3.3kΩ pull-ups) ...the system works perfectly.