2000 Toyota Sienna P1155 Code A/F Sensor

My 2000 Toyota Sienna is reading out the following code,

P1155 A/F Sensor heater Circuit Bank 2

Anyone know what this is or what could be causing it?

The car seems to run OK, except sometimes it runs rough at idle.

Thanks!

new Bee

It's the sensor on the exhaust manifold close to the radiator. Heater failed will make the sensor less functionable when start cold. Maybe related to the rough idle you mentioned.