1998 Mitsubishi Galant O2 sensor problems

The check engine light was on and a scan revealed that there was an [O2 sensor heater circuit malfunction (Bank 1, Sensor, )] which is the upstream sensor as I have been informed. That sensor has been replaced twice. I have also reset by disconecting the batery and cleared the the code with a state inspection computer. Other maintenance performed in an attempt to correct the problem includes cleaning the spark plugs and putting fuel system cleaner into gas tank. Yet the check engine light continues to come back on.