Circuit Description

The A/F sensor generates a voltage* that corresponds to the actual air-fuel ratio. This sensor voltage is used to provide the ECM with feedback so that it can control the air-fuel ratio. The ECM determines the deviation from the stoichiometric air-fuel ratio level, and regulates the fuel injection time. If the A/F sensor malfunctions, the ECM is unable to control the air-fuel ratio accurately.

The A/F sensor is the planar type and is integrated with the heater, which heats the solid electrolyte (zirconia element). This heater is controlled by the ECM. When the intake air volume is low (the exhaust gas temperature is low), a current flows into the heater to heat the sensor, in order to facilitate accurate oxygen concentration detection. In addition, the sensor and heater portions are narrower than the conventional type. The heat generated by the heater is conducted to the solid electrolyte though the alumina, therefore the sensor activation is accelerated.

In order to obtain a high purification rate of the carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) components in the exhaust gas, a TWC is used. For the most efficient use of the TWC, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric level.

*: Value changes inside the ECM. Since the A/F sensor is the current output element, a current is converted to a voltage inside the ECM. Any measurements taken at the A/F sensor or ECM connectors will show a constant voltage.

Fig 1: View Of Oxygen (A/F) Sensor And A/F Ratio And A/F Sensor Voltage Graph

Courtesy of TOYOTA MOTOR SALES, U.S.A., INC.

Fig 2: DTC Trouble Detection Chart (DTC P2195/2196)

Courtesy of TOYOTA MOTOR SALES, U.S.A., INC.

HINT:

• When any of these DTCs are set, check the A/F sensor voltage output by selecting the following menu items on a hand - held tester: DIAGNOSIS/ENHANCED OBDII/DATA LIST/ALL/AFS B1S1. If using an OBD II scan tool, refer to the instruction manual.
• Short-term fuel trim values can also be read using a hand-held tester or OBD II scan tool.
• The ECM regulates the voltage at the A1A+ and A1 A- terminals of the ECM at a constant level. Therefore, the A/F sensor voltage output cannot be confirmed without using a hand-held tester or OBD II scan tool.
• An OBD II scan tool displays the A/F sensor voltage output at a voltage level that is 1/5 of that of a hand-held tester.