Description

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 air-fuel level. For the purpose of helping the eCm to deliver accurate air-fuel ratio control, a Heated Oxygen (HO2) sensor is used.

The HO2 sensor is located behind the TWC, and detects the oxygen concentration in the exhaust gas. Since the sensor is integrated with the heater that heats the sensing portion, it is possible to detect the oxygen concentration even when the intake air volume is low (the exhaust gas temperature is low). When the air-fuel ratio becomes lean, the oxygen concentration in the exhaust gas is rich. The HO2 sensor informs the ECM that the post-TWC air-fuel ratio is lean (low voltage, i.e. less than 0.45 V). Conversely, when the air-fuel ratio is richer than the stoichiometric air-fuel level, the oxygen concentration in the exhaust gas becomes lean. The HO2 sensor informs the ECM that the post-TWC air-fuel ratio is rich (high voltage, i.e. more than 0.45 V). The HO2 sensor has the property of changing its output voltage drastically when the air-fuel ratio is close to the stoichiometric level.

The ECM uses the supplementary information from the HO2 sensor to determine whether the air-fuel ratio after the TWC is rich or lean, and adjusts the fuel injection time accordingly. Thus, if the HO2 sensor is working improperly due to internal malfunctions, the ECM is unable to compensate for deviations in the primary air-fuel ratio control.

Atmosphere Air

Ideal Air Fuel Mixture

Housing Output Voltage Platinum Electrode

Solid Electrolyte (Zirconia Element)

Heater

Coating (Ceramic)

Richer - Air Fuel Ratio - Leaner

Exhaust Gas

A115539E02

A115539E02

DTC No.

DTC Detection Conditions

Trouble Areas

P0136

• Abnormal voltage output (active air-fuel ratio control):

During active air-fuel ratio control, following conditions (a) and (b) met for certain period of time (1 trip detection logic):

(a) Heated Oxygen (HO2) sensor voltage does not decrease to less than 0.21 V

(b) HO2 sensor voltage does not increase to more than 0.59 V

Sensor impedance less than 5 Qfor more than 30 seconds when ECM presumes sensor to being warmed up and operating normally (1 trip detection logic)

• Open or short in HO2 sensor (sensor 2) circuit

• Gas leakage from exhaust system

During active air-fuel ratio control, following conditions (a) and (b) met for certain period of time (1 trip detection logic):

(a) HO2 sensor voltage output less than 0.21 V

(b) Target air-fuel ratio rich

Sensor impedance 15 kQ or more for more than 90 seconds when ECM presumes sensor to being warmed up and operating normally (1 trip detection logic)

• Open in HO2 sensor (sensor 2) circuit

• Gas leakage from exhaust system

During active air-fuel ratio control, following conditions (a) and (b) met for certain period of time (1 trip detection logic):

(a) HO2 sensor voltage output less than 0.59 V or more

(b) Target air-fuel ratio lean

HO2 sensor voltage output exceeds 1.2 V for more than 10 seconds (1 trip detection logic)

• Short in HO2 sensor (sensor 2) circuit

• ECM internal circuit malfunction

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