2001 TOYOTA SEQUOIA catalytic

DI±146
± DIAGNOSTICSENGINE
340 Author: Date:
2005 SEQUOIA (RM1146U)
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-
DIDFR±01

B17387
Active air±fuel
ratio control
OffOperation15 to 20 seconds
HO2 sensor
voltage
Abnormal Normal
0.21 V 0.59 V HO2 SENSOR CIRCUIT MALFUNCTION (P0136, P0156: ABNORMAL VOLTAGE) DI±148
± DIAGNOSTICSENGINE
342 Author: Date:
2005 SEQUOIA (RM1146U)
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.

B17388
Active air±fuel
ratio control
Target air±fuel ratio
HO2 sensor
voltageStoichiometric
Air±Fuel Level
Rich
0.21 V
Active air±fuel
ratio control
Target air±fuel ratio
HO2 sensor
voltageLean HO2 SENSOR CIRCUIT LOW VOLTAGE (P0137, P0157: OPEN)
HO2 SENSOR CIRCUIT HIGH VOLTAGE (P0138, P0158: SHORT)OffOperation
OffNormal
Abnormal 15 to 20 seconds
0.59 VAbnormalAbnormal Normal
Operation
Stoichiometric
Air±Fuel Level
± DIAGNOSTICSENGINE
DI±149
343 Author: Date:
2005 SEQUOIA (RM1146U)
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±249).

± DIAGNOSTICSENGINE
DI±249
443 Author: Date:
2005 SEQUOIA (RM1146U)
DTC P0420 Catalyst System Efficiency Below Threshold
(Bank 1)
DTC P0430 Catalyst System Efficiency Below Threshold
(Bank 2)
MONITOR DESCRIPTION
The ECM uses the two sensors, mounted in front of and behind the Three±way Catalytic Converter (TWC),
to monitor its efficiency.
The first sensor, the Air±Fuel Ratio (A/F) sensor (sensor 1), sends pre±catalyst information to the ECM. The
second sensor, the Heated Oxygen (HO2) sensor (sensor 2), sends post±catalyst information to the ECM.
The ECM compares the information transmitted by these two sensors to determine the efficiency of the TWC
performance and its ability to store oxygen.
When the TWC is functioning properly, the variation in the oxygen concentration in the exhaust gas, after
it has passed through the TWC, is small. In this condition, the voltage output of sensor 2 slowly alternates
between the rich and lean signal voltages (shown in the illustration below). As the TWC performance efficien-
cy deteriorates, its oxygen storage capacity decreases, and the variation in the oxygen concentration in the
exhaust gas increases. As a result, the sensor voltage output fluctuates frequently.
While the catalyst monitor is running, the ECM measures the signal lengths of both sensors 1 and 2, and
calculates the ratio of the signal lengths to determine the extent of the TWC deterioration. If the deterioration
level exceeds the preset threshold, the ECM interprets this as the TWC malfunction. The ECM then illumi-
nates the MIL and sets the DTC.
DTC No.DTC Detecting ConditionTrouble Area
P0420
P0430OSC value smaller than standard value under active air±fuel
ratio control (2 trip detection logic)
Gas leakage on exhaust system
A/F sensor (Bank 1, 2 sensor 1)
Heated oxygen sensor (bank 1, 2 sensor 2)
Three±way catalytic converter
HINT:
Bank 1 refers to the bank that includes cylinder No.1.
Bank 2 refers to the bank that does not include cylinder No.1.
Sensor 1 refers to the sensor mounted in front of the Three±Way Catalytic Converter (TWC) and lo-
cated near the engine assembly.
Sensor 2 refers to the sensor mounted behind the TWC and located far from the engine assembly.
DID87±01

A23542
Normal Catalyst Deteriorated Catalyst
Waveform of
A/F Sensor
in front of TWC
Waveform of
HO2 Sensor
behind TWCVoltage output when active air±fuel ratio control not performed
3.5 V
3.0 V
1.0 V
0 V
10 seconds 10 seconds DI±252
± DIAGNOSTICSENGINE
446 Author: Date:
2005 SEQUOIA (RM1146U)
HINT:
If either of the voltage outputs of the Air±Fuel Ratio (A/F) or Heated Oxygen (HO2) sensors does not
fluctuate, or either of the sensors makes a noise, the sensor may be malfunctioning.
If the voltage outputs of both the sensors remain lean or rich, the air±fuel ratio may be extremely lean
or rich. In such cases, perform the following A/F CONTROL using a hand±held tester.
If the Three±Way Catalytic Converter (TWC) has deteriorated, the HO2 sensor (located behind the
TWC) voltage output fluctuates up and down frequently, even under normal driving conditions (active
air±fuel ratio control is not performed).
A/F CONTROL
HINT:
Hand±held tester only:
Malfunctioning areas can be identified by performing the A/F CONTROL function provided in the ACTIVE
TEST. The A/F CONTROL 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 A/F CONTROL operation using a hand±held tester.
(1) Connect a hand±held tester to the DLC3.
(2) Start the engine and turn the tester ON.
(3) Warm up the engine at an engine speed of 2,500 rpm for approximately 90 seconds.
(4) On the tester, select the following menu items: DIAGNOSIS / ENHANCED OBD II / ACTIVE
TEST / A/F CONTROL.
(5) Perform the A/F CONTROL operation with the engine in an idling condition (press the RIGHT
or LEFT button to change the fuel injection volume).
(6) Monitor the voltage outputs of the A/F and HO2 sensors (AFS B1S1 and OS2 B1S2) displayed
on the tester.
HINT:
The A/F CONTROL 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.

Status
A/F S1Status
O2S S2A/F Condition and A/F and
HO2 Sensors ConditionMisfiresMain Suspected Trouble Areas
Proceed To
LeanLean/Rich Normal
± Three±way Catalytic Converter (TWC)
 Gas leakage from exhaust systemA
A A B
B
C
C Lean/Rich
Lean/Rich
Lean/Rich
Lean/Rich Lean/Rich
Lean Lean Rich
Rich
Rich
Rich LeanA/F sensor malfunction
A/F sensor malfunction
HO2 sensor malfunction
HO2 sensor malfunction
Actual air±fuel ratio lean
Actual air±fuel ratio lean
±
±
±
±
May occur A/F senso
r
 A/F sensor
 HO2 sensor
 HO2 sensor
 Gas leakage from exhaust system
 Gas leakage from exhaust system
 Gas leakage from exhaust system
 Gas leakage from exhaust system  Extremely rich or lean actual air±fuel ratio± Extremely rich or lean actual air±fuel ratio
± DIAGNOSTICSENGINE
DI±255
449 Author: Date:
2005 SEQUOIA (RM1146U)
Standard:
Tester Display
(Sensor)Injection VolumesStatusVoltages
AFS B1S1
(A/F)+25 %RichLess than 3.0
AFS B1S1
(A/F)±12.5 %LeanMore than 3.35
O2S B1S2
(HO2)+25 %RichMore than 0.55
O2S B1S2
(HO2)±12.5 %LeanLess than 0.4
RESULT:
Lean: During A/F CONTROL, the A/F sensor output voltage (AFS) is consistently more than 3.35 V, and the
HO2 sensor output voltage (O2S) is consistently less than 0.4 V.
Rich: During A/F CONTROL, the AFS is consistently less than 3.0 V, and the O2S is consistently more than
0.55 V.
B Check and replace A/F sensor.
C Check and replace heated oxygen sensor and
check and repair exhaust gas leakage.
A

DI±256
± DIAGNOSTICSENGINE
450 Author: Date:
2005 SEQUOIA (RM1146U)
3 Check gas leakage on exhaust system.
OK:
No exhaust gas leakage.
NG Repair or replace exhaust gas leakage point.
OK
Replace front and rear three±way catalytic
converter in the bank a malfunction is de-
tected.

± DIAGNOSTICSENGINE
DI±249
443 Author: Date:
2005 SEQUOIA (RM1146U)
DTC P0420 Catalyst System Efficiency Below Threshold
(Bank 1)
DTC P0430 Catalyst System Efficiency Below Threshold
(Bank 2)
MONITOR DESCRIPTION
The ECM uses the two sensors, mounted in front of and behind the Three±way Catalytic Converter (TWC),
to monitor its efficiency.
The first sensor, the Air±Fuel Ratio (A/F) sensor (sensor 1), sends pre±catalyst information to the ECM. The
second sensor, the Heated Oxygen (HO2) sensor (sensor 2), sends post±catalyst information to the ECM.
The ECM compares the information transmitted by these two sensors to determine the efficiency of the TWC
performance and its ability to store oxygen.
When the TWC is functioning properly, the variation in the oxygen concentration in the exhaust gas, after
it has passed through the TWC, is small. In this condition, the voltage output of sensor 2 slowly alternates
between the rich and lean signal voltages (shown in the illustration below). As the TWC performance efficien-
cy deteriorates, its oxygen storage capacity decreases, and the variation in the oxygen concentration in the
exhaust gas increases. As a result, the sensor voltage output fluctuates frequently.
While the catalyst monitor is running, the ECM measures the signal lengths of both sensors 1 and 2, and
calculates the ratio of the signal lengths to determine the extent of the TWC deterioration. If the deterioration
level exceeds the preset threshold, the ECM interprets this as the TWC malfunction. The ECM then illumi-
nates the MIL and sets the DTC.
DTC No.DTC Detecting ConditionTrouble Area
P0420
P0430OSC value smaller than standard value under active air±fuel
ratio control (2 trip detection logic)
Gas leakage on exhaust system
A/F sensor (Bank 1, 2 sensor 1)
Heated oxygen sensor (bank 1, 2 sensor 2)
Three±way catalytic converter
HINT:
Bank 1 refers to the bank that includes cylinder No.1.
Bank 2 refers to the bank that does not include cylinder No.1.
Sensor 1 refers to the sensor mounted in front of the Three±Way Catalytic Converter (TWC) and lo-
cated near the engine assembly.
Sensor 2 refers to the sensor mounted behind the TWC and located far from the engine assembly.
DID87±01