2003 Toyota Avalon oxygen sensor problem.?

My check engine light came on about 3 days ago so I went to Toyota. They said my bank 1 sensor 1 (upstream) air/fuel sensor needed to be replaced. They were going to charge me $500 for that. I instead went and got the oxygen senor myself and replaced it today. I got a code reader and cleared the code but the second I turned the car on again, the check engine light came on. I tried 2 more times to clear it, driving about 2 miles in between each attempt. Is the sensor still the problem or do I need a new ECM? I really don't want to go back to Toyota and have them squeeze me for more money.