Open or Short in Heated Oxygen HO Sensor Circuit DTC P or P

During active air-fuel ratio control, the ECM calculates the Oxygen Storage Capacity (OSC)* of the 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, or the voltage output of the sensor decreases significantly, 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. 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. HINT:

DTC P0138 is also set if the HO2 sensor voltage output is more than 1.2 V for 10 seconds or more.

*: The TWC has the capability to store oxygen. The OSC and the emission purification capacity of the TWC are mutually related. The ECM determines whether the catalyst has deteriorated based on the calculated OSC value (See page ES-196).

HO2 SENSOR CIRCUIT LOW VOLTAGE

15 to 20 seconds

Active air-fuel ratio control

Target air-fuel ratio

Stoichiometric Air-Fuel Level

HO2 sensor voltage

┬░peration .Abnormal

┬░peration .Abnormal

Stoichiometric Air-Fuel Level

^Normal

^Normal

Abnormal

HO2 SENSOR CIRCUIT HIGH VOLTAGE (R0138: SHORT)

Operation

Active air-fuel ratio control

... Stoichiometric

HO2 sensor voltage

Lean

Abnormal

2AZ-FE ENGINE CONTROL SYSTEM - SFI SYSTEM ES-141

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