Toyota Sequoia (2005). Manual - part 85

–

DIAGNOSTICS

ENGINE

DI–143

337

MONITOR STRATEGY

Related DTCs

P0128

Thermostat

R

i d

/

t

Main sensors/components

Engine coolant temperature sensor, Engine 
cooling system, Thermostat

Required sensors/components

Related sensors/components

Intake air temperature sensor, Vehicle speed 
sensor

Frequency of operation

Once per driving cycle

Duration

15 min.

MIL operation

2 driving cycles

Sequence of operation

None

TYPICAL ENABLING CONDITIONS

It

Specification

Item

Minimum

Maximum

The monitor will run whenever this DTC is
not present

See page 

DI–18

Battery voltage

11.0 V

–

Throttle position learning

Completed

Either of the following conditions is met

Condition 1 or 2

1. All of the following conditions are met

Condition (a), (b) and (c)

(a) ECT at engine start – IAT at engine
start

–15 to 7



C (–27 to 12.6



F)

(b) ECT at engine start

–10 to 56



C (14 to 133



F)

(c) IAT at engine start

–10 to 56



C (14 to 133



F)

2. All of the following conditions are met

Condition (a), (b) and (c)

(a) ECT at engine start – IAT at engine
start

–7



C (12.6



F)

–

(b) ECT at engine start

–

56



C (133



F)

(c) IAT at engine start

–10



C (14



F)

–

Accumulated time that vehicle speed is 80
mph (128 km/h) or more

–

20 sec.

TYPICAL MALFUNCTION THRESHOLDS

Detection Criteria

Threshold

Duration that both of following conditions 1 and 2 are met

5 sec. or more

1. Estimated engine coolant temperature

75



C (167



F) or more

2. Engine coolant temperature sensor output

Less than 75



C (167



F)

COMPONENT OPERATING RANGE

Parameter

Standard Value

Engine coolant temperature sensor output value after warm
up

75



C (167



F) or more

DI–144

–

DIAGNOSTICS

ENGINE

338

MONITOR RESULT

Refer to page 

DI–26

 for detailed information.

The test value and test limit information are described as shown in the following table. Check the monitor
result and test values after performing the monitor drive pattern (refer to ”Confirmation Monitor”).

MID (Monitor Identification Data) is assigned to each emissions–related component.

TID (Test Identification Data) is assigned to each test value.

Scaling is used to calculate the test value indicated on generic OBD ll scan tools.

Thermostat

MID

TID

Scaling

Description of Test Value

Minimum Test Limit

Maximum Test Limit

$E1

$E8

Multiply by 0.1

(

°

C)

ECT sensor output when estimated

ECT reached to malfunction criteria

Malfunction criterion

Maximum test limit

INSPECTION PROCEDURE

HINT:
Read freeze frame data using 

the hand−held tester

. Freeze frame data records the engine conditions when

a malfunction is detected. When troubleshooting, freeze frame data can help determine if the vehicle was
running or stopped, if the engine was warmed up or not, if the air–fuel ratio was lean or rich, as well as other
data from the time when a malfunction occurred.

1

Are there any other codes (besides DTC P0128) being output?

PREPARATION:
(a)

Connect the hand–held tester to the DLC3.

(b)

Turn the ignition switch ON and push the hand–held tester main switch ON.

(c)

When using hand–held tester, enter the following menu: DIAGNOSIS / ENHANCED OBD II / DTC
INFO / CURRENT CODES.

CHECK:
Read the DTC using the hand–held tester.
RESULT:

Display (DTC Output)

Proceed to

P0128

A

P0128 and other DTCs

B

HINT:
If any other codes besides P0128 are output, perform the troubleshooting for those DTCs first.

B

Go to relevant DTC chart 

(See page 

DI−58

).

A

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DIAGNOSTICS

ENGINE

DI–145

339

2

Check cooling system.

CHECK:
(a)

Check for defects in the cooling system that might cause the system to be too cold, such as abnormal
radiator fan operation or a modified cooling system.

(b)

Check the valve opening temperature of the thermostat.

OK:

Valve opening temperature is 80 to 84



C (176 to 183



F)

HINT:
Also check that the valve is completely closed under opening temperature as above.

NG

Repair or replace cooling system.

OK

3

Check thermostat (See page 

CO–12

).

CHECK:
Check the valve lift.
OK:

Valve lift: 10 mm (0.39 in.) or more at 95

°

C (203

°

F)

NG

Replace thermostat.

OK

Replace ECM (See page 

SF–80

).

DI–146

–

DIAGNOSTICS

ENGINE

340

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