Check engine light

[gabby01]

My 01 Miata SE 6 spd., gives me code p0031 on the OBDII tester. The Chilton repair manual lists this as "HO2S heater control circuit low (sensor 1). I can find no other reference or location of this sensor in the book. It also lists Several other heater control circuit sensors,
Please shed some light on where and what these sensors are.
Sincere Thanks,
gabby01