2010 Mazda 3i U0101 U3003 U0428 U0140 Fault Codes: Diagnosis and Electrical Communication Issues

I own a 2010 Mazda 3i with a 2.0L engine and have reached 150,000 miles. Recently, I've been experiencing multiple warning lights and shifting issues. The AT (Transmission) and CEL (Check Engine) lights came on while driving at 25 mph, causing erratic gear shifts—seeming to shift in and out of gears. The AT light turned off briefly, and I drove less than half a mile before stopping. At that time, the transmission fluid level was slightly above the low mark, but the fluid appeared dark. The next morning, both the AT and CEL lights were off, and the car ran normally for several hours. On day three, both warning lights returned, and gear shifts became unstable—particularly between 2nd and 3rd gears. I pulled over and had the vehicle towed. Additionally, the remote key stopped locking the doors. After replacing the battery in the remote, it still failed to function. I took the car to Capital Transmission for diagnostics over two days. They found no mechanical issues and were transparent about not charging me or recommending unnecessary repairs. They advised that a dealer diagnosis is needed to investigate communication errors between the Body Control Module (BCM) and Transmission Control Module (TCM). I drove the vehicle home, where it experienced one brief blink of the AT light during a 3-mile drive. When shifting into reverse in my driveway, both the CEL and AT lights activated. Given these symptoms, I believe this is an electrical communication issue involving multiple modules. The following diagnostic codes were pulled: - Engine: U0101 — Communication Error to TCM (Failure Type 00) - Body Control Module: U0101 — Communication Error with TCM (Failure Type 00), U3003 — Power Supply Circuit Malfunction (Failure Type 16) - Instrument Cluster Module: U0101 — Communication Error to TCM (Failure Type 00) - Anti-lock Brakes: U0428 — Abnormal Message from BCM (Failure Type 62), U0101 — CAN Line Error (Failure Type 00), U0140 — CAN Line Error (Failure Type 00) - Tire Pressure Monitoring System: U3003 — Power Supply Circuit Malfunction (Failure Type 16), U0101 — Communication Error with TCM (Failure Type 00) All codes point to recurring communication failures between the BCM and TCM via CAN bus. The repeated U0101 errors suggest a potential issue in the vehicle’s network wiring or module connectivity, possibly due to voltage drops, damaged connectors, or faulty modules. I'm seeking advice on where to begin troubleshooting—should I focus on the CAN bus harness, fuse box connections, BCM/TCM reset procedures, or consider replacing modules? Any experience with similar issues in 2010 Mazda 3i models would be greatly appreciated. Thanks, Glenn