Toyota 2GR-FE Manual page 337

MONITOR DESCRIPTION
1. Active Air-Fuel Ratio Control
The ECM usually performs air-fuel ratio feedback control so that the Air-Fuel Ratio (A/F) sensor
output indicates a near stoichiometric air-fuel level. This vehicle includes active air-fuel ratio control in
addition to regular air-fuel ratio control. The ECM performs active air-fuel ratio control to detect any
deterioration in the Three-Way Catalytic Converter (TWC) and Heated Oxygen (HO2) sensor
malfunctions (refer to the diagram below).
Active air-fuel ratio control is performed for approximately 15 to 20 seconds while driving with a warm
engine. During active air-fuel ratio control, the air-fuel ratio is forcibly regulated to become lean or rich
by the ECM. If the ECM detects a malfunction, one of the following DTCs is set: DTC P0136 or
P0156 (abnormal voltage output), P0137 or P0157 (open circuit) or P0138 or P0158 (short circuit).
2. Abnormal Voltage Output of HO2 Sensor (DTCs P0136 and P0156)
While the ECM is performing active air-fuel ratio control, the air-fuel ratio is forcibly regulated to
become rich or lean. If the sensor is not functioning properly, the voltage output variation is small. For
example, when the HO2 sensor voltage does not decrease to less than 0.21 V and does not increase
to more than 0.59 V during active air-fuel ratio control, the ECM determines that the sensor voltage
output is abnormal and sets DTCs P0136 and P0156.
HO2 SENSOR CIRCUIT MALFUNCTION (P0136 and P0156: ABNORMAL VOLTAGE):
Active air-fuel
ratio control
HO2 sensor
voltage
3. Open or Short in Heated Oxygen (HO2) Sensor Circuit (DTCs P0137 and P0157 or P0138 and
P0158)
During active air-fuel ratio control, the ECM calculates the Oxygen Storage Capacity (OSC)
Three- Way Catalytic Converter (TWC) by forcibly regulating the air-fuel ratio to become rich or lean.
If the HO2 sensor has an open or short circuit, or the voltage output of the sensor noticeably
decreases, the OSC indicates an extraordinarily high value. Even if the ECM attempts to continue
regulating the air-fuel ratio to become rich or lean, the HO2 sensor output does not change.
While performing active air-fuel ratio control, when the target air-fuel ratio is rich and the HO2 sensor
voltage output is 0.21 V or less (lean), the ECM interprets this as an abnormally low sensor output
voltage and sets DTC P0137 or P0157. When the target air-fuel ratio is lean and the voltage output is
0.59 V or more (rich) during active air-fuel ratio control, the ECM determines that the sensor voltage
output is abnormally high, and sets DTC P0138 or P0158.
HINT:
DTC P0138 or P0158 is also set if the HO2 sensor voltage output is more than 1.2 V for 30 seconds
or more.
2GR-FE ENGINE CONTROL SYSTEM – SFI SYSTEM
Off
Normal
0.59 V
Abnormal
0.21 V
15 to 20 seconds
Operation
ES–181
ES
A076837E10
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of the