Circuit Description

WARNING: This page is about a different car, the 2004 Lexus ES 330. However, it is still accessible from the selected car via links, so may be relevant.

The heated oxygen sensor is used to monitor oxygen concentration in the exhaust. For optimum catalytic converter operation, the air-fuel mixture must be maintained near the ideal stoichiometric air-fuel ratio. The heated oxygen sensor output voltage changes at the stoichiometric ratio. The ECM adjusts the fuel injection time so that the air-fuel ratio is nearly stoichiometric.

If the oxygen concentration in the exhaust gas increases, the air-fuel ratio is called LEAN. The heated oxygen sensor voltage drops below 0.45V, which informs the ECM of the LEAN condition.

If oxygen is not in the exhaust gas, the air-fuel ratio is called RICH. The heated oxygen sensor voltage increases above 0.45V, which informs the ECM of the RICH condition.

The heated oxygen sensors include a heater which heats the zirconia element. The heater is controlled by the ECM. When the intake air volume is low (the temperature of the exhaust gas is low), current flows to the heater in order to heat the sensor for the accurate oxygen concentration detection.

Fig 1: Heated Oxygen Sensor Circuit Description Chart

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Fig 2: DTC P0138 And P0156 Diagnostic Chart

HINT:

Bank 1 refers to the bank that includes cylinder No.1.
Bank 2 refers to the bank that does not include cylinder No.1.
Sensor 2 refers to the sensor farthest away from the engine assembly.