Tacoma Calif. Emission Vehicles & 4Runner Calif. Emission Vehicles

Heated A/F sensor is mounted on exhaust pipe in front of catalytic converter. If problem exists in A/F sensor, heater or wiring circuit, a Diagnostic Trouble Code (DTC) may be stored in Engine Control Module (ECM). See SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article for retrieving and servicing of DTCs.
To test heater resistance on A/F sensor, disconnect electrical connector for A/F sensor. Using ohmmeter, check resistance between +B and HT terminals on electrical connector. See Fig 1 and Fig 2 . Replace A/F sensor if resistance is not.8-1.4 ohms at 68°F (20°C) and 1.8-3.2 ohms at 1472°F (800°C).

Fig 1: Testing Heater On A/F Sensor (Tacoma Calif. Emission Vehicles)
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Fig 2: Testing Heater On A/F Sensor (4Runner Calif. Emission Vehicles)
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002