Monitor Description

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 (abnormal voltage output), P0137 (open circuit) and P0138 (short circuit).

Abnormal Voltage Output of HO2 Sensor (DTC P0136)

• 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 DTC P0136.

HO2 SENSOR CIRCUIT MALFUNCTION (R0136: ABNORMAL VOLTAGE)

15 to 20 seconds

Operation

Active air fuel ratio control

HO2 sensor voltage

Operation

' \ / \

Normal

----

Abnormal

--^-1-

Open or Short in Heated Oxygen (HO2) Sensor Circuit (DTC P0137 or P0138)

• 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 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. 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-206).

HO2 SENSOR CIRCUIT LOW VOLTAGE (P0137: OPEN)

15 to 20 seconds

Active air fuel ratio control

Target air fuel ratio

HO2 sensor voltage

Stoichiometric Air Fuel Level

15 to 20 seconds

Stoichiometric Air Fuel Level

Operation

Abnormal n in n

Normal^

i__1 / L.J

^Normal

Rich r\ r ' i ! 1 !

V J w

Abnormal

HO2 SENSOR CIRCUIT HIGH VOLTAGE (R01S8: SHORT)

Operation

Active air fuel ratio control

Target air fuel ratio

HO2 sensor voltage

Stoichiometric Air Fuel Level

Abnormal

Operation

Stoichiometric Air Fuel Level

Abnormal

High or Low Impedance of Heated Oxygen (HO2) Sensor (DTC P0136 or P0137)

During normal air-fuel ratio feedback control, there are small variations in the exhaust gas oxygen concentration. In order to continuously monitor the slight variation of the HO2 sensor signal while the engine is running, the impedance* of the sensor is measured by the ECM. The ECM determines that there is a malfunction in the sensor when the measured impedance deviates from the standard range.

*: The effective resistance in an alternating current electrical circuit. HINT:

• The impedance cannot be measured using an ohmmeter.

• DTC P0136 indicates the deterioration of the HO2 sensor. The ECM sets the DTC by calculating the impedance of the sensor when the typical enabling conditions are satisfied (1 driving cycle).

• DTC P0137 indicates an open or short circuit in the HO2 sensor (1 driving cycle). The ECM sets the DTC when the impedance of the sensor exceeds the threshold 15 kQ.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Post a comment