1996 Cadillac Eldorado 32 Valve Northstar O2 Sensor P0143 Fault Diagnosis and Fix

Hello, I recently purchased a 1996 Cadillac Eldorado with a 32-valve Northstar engine. It's a 2-door coupe with front-wheel drive and has accumulated 108,000 miles. The car was originally driven from Medford, Oregon to Santa Rosa, California — a journey that included winding mountain roads with elevation peaks up to 3,000 feet. After arriving in California, the check engine light came on, displaying 'Check Emissions Control System' on the dashboard. Despite this, the vehicle runs smoothly and has no noticeable performance issues. I used an OBD2 scanner at home and found fault code P0143: 'O2 Circuit Low Voltage - Bank 1, Sensor 3'. This is a downstream oxygen sensor located after the catalytic converter. I have already purchased a replacement sensor but haven't installed it yet. No other warning lights are active, and no additional diagnostic codes appear. The car continues to run normally even with the check engine light on. I'm seeking advice on whether this P0143 code is likely caused by a faulty oxygen sensor or another underlying issue, especially given the vehicle's age and mileage. Any insights into potential causes or next steps would be greatly appreciated.