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Home >> Toyota >> 2001 >> Sienna CE >> Repair and Diagnosis >> External Pages >> Different car >> Section 99 (Engine Controls - Self-Diagnostics) >> Diagnostic Tests >> DTC P0031, DTC P0032, DTC P0037, DTC P0038, DTC P0051, DTC P0052, DTC P0057 Or DTC P0058 >> Circuit Description

Circuit Description

WARNING: This page is about a different car, the 2003 Lexus LX 470. However, it is still accessible from the selected car via links, so may be relevant.

To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a 3-way catalytic converter is used, but for the most efficient use of the 3-way catalytic converter, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric air-fuel ratio.

The heated oxygen sensor has the characteristic which its output voltage changes suddenly in the vicinity he stoichiometric air-fuel ratio. This characteristic is used to detect the oxygen concentration in the exhaust gas and provide the ECM with feedback to control the air-fuel ratio.

When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust increases and the heated oxygen sensor informs the ECM of the LEAN condition (small electromotive force: less than 0.45 volt). When the air-fuel ratio is RICHER than the stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas is reduced and the heated oxygen sensor informs the ECM of the RICH condition (large electromotive force: greater than 0.45 volt). The ECM judges by the electromotive force from the heated oxygen sensor whether the air-fuel ratio is RICH or LEAN and controls the injection time accordingly. However, if malfunction of the heated oxygen sensor causes output of abnormal electromotive force, the ECM is unable to perform accurate air-fuel ratio control. 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 to heat the sensor for accurate oxygen concentration detection. See Fig 1.

DTCs P0031, P0037, P0051 and P0057 are set when heater current of 0.2 amps or less is present when heater operates with +B at greater than 10.5 volts and less than 11.5 volts (one trip detection logic) or heater current of 0.25 amps or less when heater operates with +B at greater than 11.5 volts (one trip detection logic). DTCs P0032, P0038, P0052 and P0058 are set when heater operates, heater current exceeds 2 amps (one trip detection logic). Possible causes are:

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