2000 Chevrolet Metro P0136: Oxygen Sensor Circuit Malfunction - Diagnosis and Fix

First post — please be gentle with feedback. I own a 2000 Chevrolet Metro with a 1.3L automatic transmission. Recently, the Check Engine Light (CEL) has come on. My OBD-II scanner displays fault code P0136: "Oxygen Sensor Circuit Malfunction (Bank 1, Sensor 2)". Using EasyOBDII software to monitor live data while running the engine, I notice that the rear oxygen sensor consistently reads 1.275V and does not fluctuate — it remains stable. Even when I disconnect its connector, the scanner still shows 1.275V. In contrast, the front oxygen sensor appears to be functioning properly. It rapidly varies between 0.2V and 0.9V. When I disconnect that sensor, the scanner reads 0V with a rich fuel condition, indicating it is working correctly. Given this behavior, I suspect the rear O2 sensor may be faulty or has an internal circuit issue. However, since the reading remains constant even when disconnected, it suggests either a wiring problem, a defective sensor, or a potential ECU misreading. Has anyone else experienced this P0136 code on their 2000 Chevrolet Metro? What was the root cause and how was it resolved? Thank you for any insights or advice.