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DI-567
DIAGNOSTICS - ENGINE (2UZ-FE)
DIDRE-01
DTC P0136 Oxygen Sensor Circuit Malfunction (Bank 1
Sensor 2)
DTC P0137 Oxygen Sensor Circuit Low Voltage (Bank 1
Sensor 2)
DTC P0138 Oxygen Sensor Circuit High Voltage (Bank 1
Sensor 2)
DTC P0156 Oxygen Sensor Circuit Malfunction (Bank 2
Sensor 2)
DTC P0157 Oxygen Sensor Circuit Low Voltage (Bank 2
Sensor 2)
DTC P0158 Oxygen Sensor Circuit High Voltage (Bank 2
Sensor 2)
HINT:
Sensor 2 refers to the sensor mounted behind the Three-Way Catalytic Converter (TWC) and located far
from the engine assembly.
CIRCUIT 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 drasti-
cally 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 work-
ing improperly due to internal malfunctions, the ECM is unable to compensate for deviations in the primary
air-fuel ratio control.
DI-568
DIAGNOSTICS - ENGINE (2UZ-FE)
Atmospheric Air
Ideal Air-Fuel Mixture
Housing
Output Voltage
Solid Electrolyte
(Zirconia Element)
Platinum Electrode
Heater
Coating (Ceramic)
Cover Richer - Air Fuel Ratio - Leaner
Exhaust Gas
B17386
DTC No. DTC Detecting Condition Trouble Area
During active air-fuel ratio control, following conditions (a) and
(b) met for certain period of time (2 trip detection logic):
P0136 S Open or short in HO2 sensor (sensor 2) circuit
(a) Heated Oxygen (HO2) sensor voltage does not decrease to S HO2 sensor (sensor 2)
P0156
less than 0.2 V S HO2 sensor heater (sensor 2)
(b) HO2 sensor voltage does not increase to more than 0.6 V S Air-Fuel Ratio (A/F) sensor (sensor 1)
Air Fuel
Sensor impedance less than 5 for more than 30 seconds S EFI relay
P0136 S Gas leakage from exhaust system
when ECM presumes sensor to being warmed up and operat-
P0156
ing normally (1 trip detection logic)
During active air-fuel ratio control, following conditions (a) and
P0137 (b) met for certain period of time (2 trip detection logic):
P0157 (a) HO2 sensor voltage output less than 0.21 V S Open in HO2 sensor (sensor 2) circuit
(b) Target air-fuel ratio rich S HO2 sensor (sensor 2)
S HO2 sensor heater (sensor 2)
High impedance:
S EFI relay
P0137 Sensor impedance 348.1 M or more for more than 90 se-
S Gas leakage from exhaust system
P0157 conds when ECM presumes sensor to being warmed up and
operating normally (1 trip detection logic)
During active air-fuel ratio control, following conditions (a) and
P0138 (b) met for certain period of time (2 trip detection logic):
P0158 (a) HO2 sensor voltage output 0.59 V or more S Short in HO2 sensor (sensor 2) circuit
(b) Target air-fuel ratio lean S HO2 sensor (sensor 2)
S ECM internal circuit malfunction
P0138 HO2 sensor voltage output exceeds 1.2 V for more than 30
P0158 seconds (1 trip detection logic)
DI-569
DIAGNOSTICS - ENGINE (2UZ-FE)
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 indi-
cates a near stoichiometric air-fuel level. This vehicle includes active air-fuel ratio control in addition to regu-
lar 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, P0156 (abnormal voltage
output), P0137, P0157 (open circuit) and P0138, P0158 (short circuit).
Abnormal Voltage Output of HO2 Sensor (DTC P0136, P0156)
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 (P0136, P0156: ABNORMAL VOLTAGE)
15 to 20 seconds
Operation
Active air-fuel
ratio control Off
Normal
0.59 V
HO2 sensor
voltage
Abnormal
0.21 V
B17387
DI-570
DIAGNOSTICS - ENGINE (2UZ-FE)
Open or Short in the Heated Oxygen (HO2) Sensor Circuit (DTC P0137, P0157, P0138 or 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 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 or P0157. 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 or P0158.
HINT:
DTC P0138 or P0158 is also set if the HO2 sensor voltage output is more than 1.2 V for 30 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 DI-672).
HO2 SENSOR CIRCUIT LOW VOLTAGE (P0137, P0157: OPEN)
15 to 20 seconds
Operation
Active air-fuel
ratio control Off Normal Abnormal
Stoichiometric
Target air-fuel ratio Air-Fuel Level
Normal
Rich
HO2 sensor
voltage
0.21 V
Abnormal
HO2 SENSOR CIRCUIT HIGH VOLTAGE (P0138, P0158: SHORT)
Operation
Active air-fuel
ratio control Off
Lean
Stoichiometric
Target air-fuel ratio
Air-Fuel Level Abnormal
HO2 sensor 0.59 V
voltage
B17388
DI-571
DIAGNOSTICS - ENGINE (2UZ-FE)
High or Low Impedance of Heated Oxygen (HO2) Sensor (DTC P0136, P0156, P0137 or P0157)
During normal air-fuel ratio feedback control, there are small
Interrelation between temperature of
the element and impedance: variations in the exhaust gas oxygen concentration. In order to
continuously monitor the slight variation of the HO2 sensor sig-
DTC Detection Area nal while the engine is running, the impedance* of the sensor
is measured by the ECM. The ECM determines that there is a
15,000 malfunction in the sensor when the measured impedance devi-
Impedance
1,000 ates from the standard range.
*: The effective resistance in an alternating current electrical cir-
100
cuit.
10 HINT:
5
S The impedance can not be measured using an ohmme-
300
(572)
400
(752)
500
(932)
600 700
(1,112) (1,292)
800
(1,472)
ter.
S DTC P0136 or P0156 indicates the deterioration of the
Temperature _C (_F)
B17389 HO2 sensor. The ECM sets the DTC by calculating the im-
pedance of the sensor when the typical enabling condi-
tions are satisfied (1 driving cycle).
S DTC P0137 or P0157 indicates an open circuit in the HO2
sensor (1 driving cycle). The ECM sets this DTC when the
impedance of the sensor exceeds the threshold 348.1
M.
DI-572
DIAGNOSTICS - ENGINE (2UZ-FE)
MONITOR STRATEGY
Heated rear oxygen sensor (Bank 1) output volt-
age (Output voltage)
P0136
Heated rear oxygen sensor (Bank 1) impedance
(Low)
Heated rear oxygen sensor (Bank 1) output volt-
age (Low voltage)
P0137
Heated rear oxygen sensor (Bank 1) impedance
(High)
Heated rear oxygen sensor (Bank 1) output volt-
age (High voltage)
P0138
Heated rear oxygen sensor (Bank 1) output volt-
age (Extremely high)
Related DTC
R l t d DTCs
Heated rear oxygen sensor (Bank 2) output volt-
age (Output voltage)
P0156
Heated rear oxygen sensor (Bank 2) impedance
(Low)
Heated rear oxygen sensor (Bank 2) output volt-
age (Low voltage)
P0157
Heated rear oxygen sensor (Bank 2) impedance
(High)
Heated rear oxygen sensor (Bank 2) output volt-
age (High voltage)
P0158
Heated rear oxygen sensor (Bank 2) output volt-
age (Extremely high)
Main sensors/components Heated rear oxygen sensor
Required sensors/components
R i d / t
Related sensors/components Mass air flow meter
Once per driving cycle: Active air-fuel ratio control detection
Frequency of operation
Continuous: Others
20 sec.: Heated oxygen sensor output (Output voltage, High voltage, Low voltage)
30 sec.: Heated oxygen sensor impedance (Low)
Duration
90 sec.: Heated oxygen sensor impedance (High)
10 sec.: Heated oxygen sensor output (Extremely high)
2 driving cycles Heated oxygen sensor output (Output voltage, High voltage, Low voltage, Extremely
MIL operation high)
Immediate: Heated oxygen sensor impedance (Low, High)
Sequence of operation None
TYPICAL ENABLING CONDITIONS
Specification
Item
It
Minimum Maximum
The monitor will run whenever this DTC is
See page DI-437
not present
Heated oxygen sensor output voltage (Output voltage, High voltage and Low voltage):
Active air-fuel ratio control Performing
Active air-fuel ratio control being when all
-
of following conditions met
Battery voltage 11 V -
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