A/F Sensor Heater

Turn OFF ignition switch and disconnect A/F sensor connector.
Check that voltage of A/F sensor connector (vehicle side) between terminals "4" and "3" is battery voltage when ignition switch is in ON position.
If faulty condition is found, check power supply circuit, ground circuit and connections.

Fig 1: Identifying A/F Sensor Heater Connector Terminal
Courtesy of SUZUKI OF AMERICA CORP.
Using ohmmeter, measure resistance of A/F sensor connector (sensor side) between terminals "3" and "4" at A/F sensor connector. If faulty condition is found, replace A/F sensor.
NOTE: Sensor temperature affects resistance value largely. Make sure that sensor heater is at specified temperature.

A/F sensor heater resistance

2.16 - 2.64 Ω at 20 °C (68 °F)

Fig 2: Identifying A/F Sensor Heater Connector Terminal
Courtesy of SUZUKI OF AMERICA CORP.