O2 Sensor?

All,
I have a 2003 Tahoe 5.3 with 74K. Yesterday the engine light came on and code P1133 was set. I understand this truck has 4 o2 sensors. I the heated 02 sensor the one just after the
cat?
thanks! John
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All the sensors are heated, code says #1 sensor has insufficient switching, a full diagnostic scan is required here.

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