Toyota Tundra. Manual — part 1352

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
or P0156.

Fig. 82: Active Air-Fuel Ratio Control Graph

Open or Short in Heated Oxygen (HO2) Sensor Circuit (DTC P0137 and P0157 or P0138 and P0158)

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 or P0157. When the target air-fuel ratio is lean and the voltage output is 0.59 V or higher (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 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

DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1); DTC P0430

CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 2)

).

2008 Toyota Tundra

2008 ENGINE PERFORMANCE Engine Control System (2UZ-FE) - Tundra

Fig. 83: (HO2) Sensor Low & High Voltage Graph

High or Low Impedance of Heated Oxygen (HO2) Sensor (DTC P0136 and P0156 or P0137 and P157)

2008 Toyota Tundra

2008 ENGINE PERFORMANCE Engine Control System (2UZ-FE) - Tundra

Fig. 84: Identifying DTC Detection Area

During normal air-fuel ratio feedback control, there are small variations in the exhaust gas oxygen
concentration. In order to continuously monitor the slight variations in 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 and P0156 indicate the deterioration of the HO2 sensor. The ECM sets this DTC by
calculating the impedance of the sensor when the typical enabling conditions are satisfied (2 driving
cycles).



DTC P0137 and P0157 indicate an open or short circuit in the HO2 sensor (2 driving cycles). The ECM
sets this DTC when the impedance of the sensor exceeds the threshold 15 kohms.

HO2 sensor output voltage during fuel cut (P0139 or P0159)

The sensor voltage drops below 0.2 V (extremely lean status) immediately when the vehicle decelerates and
fuel cut is operating. If the voltage does not drop below 0.2 V, the ECM determines that the sensor response
feature has deteriorated, illuminates the MIL and stores DTC (s).

MONITOR STRATEGY

MONITOR STRATEGY

P0136: Heated oxygen sensor (for bank 1) output voltage (Abnormal
voltage output)
P0136: Heated oxygen sensor (for bank 1) impedance (Low)
P0137: Heated oxygen sensor (for bank 1) output voltage (Low voltage)
P0137: Heated oxygen sensor (for bank 1) impedance (High)

2008 Toyota Tundra

2008 ENGINE PERFORMANCE Engine Control System (2UZ-FE) - Tundra

TYPICAL ENABLING CONDITIONS

All:

TYPICAL ENABLING CONDITIONS - ALL

Related DTCs

P0138: Heated oxygen sensor (for bank 1) output voltage (High
voltage)
P0138: Heated oxygen sensor (for bank 1) output voltage (Extremely
high)
P0139: Heated oxygen sensor output Voltage during fuel cut (for Bank
1)
P0156: Heated oxygen sensor (for bank 2) output voltage (Abnormal
voltage output)
P0156: Heated oxygen sensor (for bank 2) impedance (Low)
P0157: Heated oxygen sensor (for bank 2) output voltage (Low voltage)
P0157: Heated oxygen sensor (for bank 2) impedance (High)
P0158: Heated oxygen sensor (for bank 2) output voltage (High
voltage)
P0158: Heated oxygen sensor (for bank 2) output voltage (Extremely
high)
P0159: Heated oxygen sensor output voltage during fuel cut (for Bank
2)

Required Sensors/Components
(Main)

Heated oxygen sensor

Required Sensors/Components
(Related)

Crankshaft position sensor, engine coolant temperature sensor, mass air
flow meter and throttle position sensor

Frequency of Operation

Once per driving cycle: Active air-fuel ratio control detection
Continuous: Other

Duration

20 seconds: Active air-fuel ratio control detection
90 seconds: Heated oxygen sensor impedance (High)
30 seconds: Heated oxygen sensor impedance (Low)
10 seconds: Output voltage (Stuck high)
More than 9.3 seconds: Output voltage (During fuel cut)

MIL Operation

2 driving cycles

Sequence of Operation

None

Monitor runs whenever following DTCs not

P0031, P0032, P0051, P0052 (A/F Sensor Heater -
Sensor 1)
P0037, P0038, P0057, P0058 (O2 Sensor Heater -
Sensor 2)
P0102 - P0103 (MAF meter)
P0110 - P0113 (IAT sensor)
P0115 - P0118 (ECT sensor)
P0120 - P0223, P2135 (TP sensor)
P0125 (Insufficient ECT for Closed Loop)
P0171, P0172 (Fuel system)
P0300 - P0308 (Misfire)

2008 Toyota Tundra

2008 ENGINE PERFORMANCE Engine Control System (2UZ-FE) - Tundra