SFI SYSTEM(w/ Secondary Air Injection System), Diagnostic DTC:P0137, P0157

DTC Code	DTC Name
P0137	Oxygen Sensor Circuit Low Voltage (Bank 1 Sensor 2)
P0157	Oxygen Sensor Circuit Low Voltage (Bank 2 Sensor 2)

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 ratio. 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. below 0.45 V).

Conversely, when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, 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. higher 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.

DTC Code	DTC Detection Condition	Trouble Area
P0137 P0157	Low voltage (open): During active air-fuel ratio control, the following conditions (a) and (b) are met for a certain period of time (2 trip detection logic): The HO2 sensor voltage output is below 0.21 V. The target air-fuel ratio is rich.	Open in HO2 sensor (for Bank 1, 2) circuit HO2 sensor (for Bank 1, 2) HO2 sensor heater (for Bank 1, 2) No. 2 integration relay Gas leakage from exhaust system

DTC Code

DTC Detection Condition

Trouble Area

P0137

P0157

Low voltage (open):

During active air-fuel ratio control, the following conditions (a) and (b) are met for a certain period of time (2 trip detection logic):

The HO2 sensor voltage output is below 0.21 V.
The target air-fuel ratio is rich.

Open in HO2 sensor (for Bank 1, 2) circuit
HO2 sensor (for Bank 1, 2)
HO2 sensor heater (for Bank 1, 2)
No. 2 integration relay
Gas leakage from exhaust system

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 ratio. 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, a DTC is stored.

Open or Short in Heated Oxygen (HO2) Sensor Circuit (DTC P0137 and P0157)
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 stores DTC P0137 or P0157.

*: 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.

WIRING DIAGRAM

CONFIRMATION DRIVING PATTERN

Tech Tips

This confirmation driving pattern is used in the "Perform Confirmation Driving Pattern" procedure of the following diagnostic troubleshooting procedure.
Performing this confirmation driving pattern will activate the Heated Oxygen (HO2) sensor monitor (the catalyst monitor is performed simultaneously). This is very useful for verifying the completion of a repair.

Note

This test will not be completed if the vehicle is driven under absolutely constant speed conditions such as with cruise control activated.

Connect the intelligent tester to the DLC3.
Turn the engine switch on (IG).
Turn the tester on.
Enter the following menus: Powertrain / Engine and ECT / DTC / Clear.
Clear DTCs.
Switch the ECM from normal mode to check mode
Enter the following menus: Powertrain / Engine and ECT / Data List / O2S (A/FS) Monitor.
Check that the item O2S (A/FS) Monitor displays Incmpl (Incomplete).
Start the engine and warm it up. (Proceed to "A")
Drive the vehicle at between 90 km/h and 110 km/h (56 mph and 68 mph) for at least 10 minutes. (Proceed to "B")
Note the state of the Data List items. Those items will change to Compl (Complete) as the O2 Sensor monitor operates.
On the tester, enter the following menus: Powertrain / Engine and ECT / DTC / Pending and check if any DTCs (any pending DTCs) are output.

CAUTION / NOTICE / HINT

Tech Tips

Malfunctioning areas can be identified by performing the Control the Injection Volume for A/F sensor function provided in the Active Test. The Control the Injection Volume for A/F sensor function can help to determine whether the Air Fuel ratio (A/F) sensor, Heated Oxygen (HO2) sensor and other potential trouble areas are malfunctioning.

The following instructions describe how to conduct the Control the Injection Volume for A/F sensor operation using the intelligent tester.

Connect the tester to the DLC3.
Start the engine and turn the tester on.
Warm up the engine at an engine speed of 2500 rpm for approximately 90 seconds.
Enter the following menus: Powertrain / Engine and ECT / Active Test / Control the Injection Volume for A/F sensor.
Perform the Active Test operation with the engine in an idling condition (press the RIGHT or LEFT button to change the fuel injection volume).
Monitor the output voltages of the A/F and HO2 sensors (AFS B1 S1 and O2S B1 S2 or AFS B2 S1 and O2S B2 S2) displayed on the tester.

Tech Tips

The Control the Injection Volume for A/F sensor operation lowers the fuel injection volume by 12.5% or increases the injection volume by 25%.
Each sensor reacts in accordance with increases and decreases in the fuel injection volume.

Display Item (Sensor)	Injection Volume	Status	Voltage
AFS B1 S1 or AFS B2 S1 (A/F)	+25%	Rich	Below 3.1 V
AFS B1 S1 or AFS B2 S1 (A/F)	-12.5%	Lean	Higher than 3.4 V
O2S B1 S2 or O2S B2 S2 (HO2)	+25%	Rich	Higher than 0.55 V
O2S B1 S2 or O2S B2 S2 (HO2)	-12.5%	Lean	Below 0.4 V

Note

The Air Fuel ratio (A/F) sensor has an output delay of a few seconds and the Heated Oxygen (HO2) sensor has a maximum output delay of approximately 20 seconds.

Following the Control the Injection Volume for A/F sensor procedure enables technicians to check and graph the voltage outputs of both the A/F and HO2 sensors.
To display the graph, enter the following menus: Powertrain / Engine and ECT / Active Test / Control the Injection Volume / All Data / AFS Voltage B1S1 and O2S B1S2 or AFS Voltage B2S1 and O2S B2S2.

Tech Tips

Read freeze frame data using the intelligent tester. Freeze frame data records the engine condition when malfunctions are detected. When troubleshooting, freeze frame data can help determine if the vehicle was moving or stationary, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other data from the time the malfunction occurred.
Bank 1 refers to the bank that includes the No. 1 cylinder*.

*: The No. 1 cylinder is the cylinder which is farthest from the transmission.
Bank 2 refers to the bank that does not include the No. 1 cylinder.
Sensor 1 refers to the sensor closest to the engine assembly.
Sensor 2 refers to the sensor farthest away from the engine assembly.

PROCEDURE

READ OUTPUT DTC

Connect the intelligent tester to the DLC3.
Turn the engine switch on (IG) and turn the tester on.
Enter the following menus: Powertrain / Engine and ECT / DTC.

Read DTCs.

Result
Result	Proceed to
P0137 or P0157 is output	A
P0137 or P0157 and other DTCs are output	B

GO TO DTC CHART Click here

CHECK FOR EXHAUST GAS LEAKAGE

OK

No gas leakage.

NG
REPAIR OR REPLACE EXHAUST GAS LEAKAGE POINT

OK

OK
No gas leakage.

INSPECT HEATED OXYGEN SENSOR (HEATER RESISTANCE)


*1	Component without harness connected (HO2 Sensor for Bank 1)
*2	Component without harness connected (HO2 Sensor for Bank 2)

Disconnect the C20 or C56 heated oxygen (HO2) sensor connector.

Measure the resistance according to the value(s) in the table below.

Standard Resistance

Tester Connection	Condition	Specified Condition
1 (HT1B) - 2 (+B)	20°C (68°F)	11 to 16 Ω
1 (HT1B) - 4 (E1)	Always	10 kΩ or higher
1 (HT2B) - 2 (+B)	20°C (68°F)	11 to 16 Ω
1 (HT2B) - 4 (E1)	Always	10 kΩ or higher

REPLACE HEATED OXYGEN SENSOR Click here

INSPECT NO. 2 INTEGRATION RELAY (EFI MAIN NO. 1)
1. Remove the No. 2 integration relay from the engine room relay block.
2. Inspect the No. 2 integration relay (EFI MAIN NO. 1) Click here.
NG
REPLACE NO. 2 INTEGRATION RELAY

OK

CHECK HARNESS AND CONNECTOR (HEATED OXYGEN SENSOR - ECM)

Disconnect the C20 or C56 HO2 sensor connector.

Measure the voltage according to the value(s) in the table below.

Standard Voltage

Tester Connection	Switch Condition	Specified Condition
C20-2 (+B) - Body ground	Engine switch on (IG)	11 to 14 V
C56-2 (+B) - Body ground	Engine switch on (IG)	11 to 14 V

Turn the engine switch off.
Disconnect the C45 ECM connector.

Measure the resistance according to the value(s) in the table below.

Standard Resistance

Tester Connection	Condition	Specified Condition
C20-1 (HT1B) - C45-21 (HT1B)	Always	Below 1 Ω
C20-3 (OX1B) - C45-96 (OX1B)	Always	Below 1 Ω
C20-4 (E1) - C45-95 (EX1B)	Always	Below 1 Ω
C56-1 (HT2B) - C45-44 (HT2B)	Always	Below 1 Ω
C56-3 (OX2B) - C45-119 (OX2B)	Always	Below 1 Ω
C56-4 (E1) - C45-118 (EX2B)	Always	Below 1 Ω
C20-1 (HT1B) or C45-21 (HT1B) - Body ground	Always	10 kΩ or higher
C20-3 (OX1B) or C45-96 (OX1B) - Body ground	Always	10 kΩ or higher
C56-1 (HT2B) or C45-44 (HT2B) - Body ground	Always	10 kΩ or higher
C56-3 (OX2B) or C45-119 (OX2B) - Body ground	Always	10 kΩ or higher

REPAIR OR REPLACE HARNESS OR CONNECTOR

REPLACE HEATED OXYGEN SENSOR
1. Replace the heated oxygen sensor Click here.
NEXT

CHECK WHETHER DTC OUTPUT RECURS

Connect the intelligent tester to the DLC3.
Turn the engine switch on (IG).
Turn the intelligent tester on.
Enter the following menus: Powertrain / Engine and ECT / Utility / All Readiness.
Input DTCs: P0137 or P0157.

Check that DTC MONITOR is NORMAL. If DTC MONITOR is INCOMPLETE, perform the drive pattern adding the vehicle speed.

Result
Result	Proceed to
NORMAL (No DTC output)	A
ABNORMAL (P0137 or P0157 detected)	B

END

REPLACE AIR FUEL RATIO SENSOR Click here