I've a "P0133: O2 sensor circuit slow response bank 1 sensor 1" dtc. Is there a good chance the sensor has gone bad, or could it be something else?

Found the answer here. Could be an exhaust leak.

The "P0133" code showed up in my '98 Windstar last summer. I replaced both upstream oxygen sensors and cleared the code. That fixed the problem for me and the van is running much better now.