P0134 After ECU Replacement in 1998 Hyundai Elantra - O2 Sensor Circuit No Activity Detected

Hi everyone, I'm having a persistent issue with my 1998 Hyundai Elantra (VIN: KMHJW34M0WU). The problem started with black smoke from the exhaust and noticeable lack of power. After taking it to a repair shop, they diagnosed the timing belt and oxygen sensor as needing replacement based on OBDII scan results. However, after replacing both components, symptoms persisted. The technician then suggested replacing the MAF sensor, claiming this would resolve the black smoke — it did, but the lack of power issue remained. Subsequently, we replaced spark plugs, fuel filter, fuel pump, air filter, and checked spark plug wires. An oil change with new oil filter was also performed, along with cleaning the air intake system. Despite these efforts, performance issues continued. The shop finally concluded that the ECU (Engine Control Unit) was faulty and recommended replacing it. After installation, the car ran for a few miles before the check engine light came back on. Using an OBDII scanner, I now see error code P0134: 'O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 1)' — even though the oxygen sensor was already replaced earlier. Any insights or suggestions on what might be causing this? Is it possible that the ECU replacement introduced a new fault, or could there be an underlying issue with the wiring or sensor circuit?