Toyota Sequoia (2005). Manual - part 91

B17394

Fuel
Compensation 
Amount

1.35

1.0

0.65

+35 (%):
Lean Malfunction Threshold

–35 (%):
Rich Malfunction Threshold

P0171, P0174:

P0172, P0175:

–

DIAGNOSTICS

ENGINE

DI–167

361

MONITOR DESCRIPTION

Under closed–loop fuel control, fuel injection volumes that deviate from those estimated by the ECM cause
changes in the long–term fuel trim compensation value. The long–term fuel trim is adjusted when there are
persistent deviations in the short–term fuel trim values. Deviations from the ECM’s estimated fuel injection
volumes also affect the average fuel trim learning value, which is a combination of the average short–term
fuel trim (fuel feedback compensation value) and the average long–term fuel trim (learning value of the air–
fuel ratio). If the average fuel trim learning value exceeds the malfunction thresholds, the ECM interprets
this a fault in the fuel system and sets a DTC.
Example:
The average fuel trim leaning value is more than +35 % or less than –35 %, the ECM interprets this as a fuel
system malfunction.

MONITOR STRATEGY

P0171

Fuel system lean (Bank 1)

R l t d DTC

P0172

Fuel system rich (Bank 1)

Related DTCs

P0174

Fuel system lean (Bank 2)

P0175

Fuel system rich (Bank 2)

Main sensors/components

Front oxygen sensor

Required sensors/components

Related sensors/components

Engine coolant temperature sensor, Mass air flow
meter, Crankshaft position sensor

Frequency of operation

Continuous

Duration

10 sec.

MIL operation

2 driving cycles

Sequence of operation

None

DI–168

–

DIAGNOSTICS

ENGINE

362

TYPICAL ENABLING CONDITIONS

It

Specification

Item

Minimum

Maximum

The monitor will run whenever these

DTCs are not present

See page 

DI–18

Battery voltage

11 V

–

Fuel system status

Closed–loop

Throttle position learning

Completed

Either of the following conditions is met:

Condition 1 or 2

1. Engine RPM

–

1,000 rpm

2. Intake air amount per revolution

0.26 g/sec.

–

Catalyst monitor

No executed

TYPICAL MALFUNCTION THRESHOLDS

Detection Criteria

Threshold

Purge–cut

Executing

Either of the following conditions is met

Condition 1 or 2

1. Average between short–term fuel trim and long–term fuel
trim

35% or more (varies with ECT)

2. Average between short–term fuel trim and long–term fuel
trim

–35% or less (varies with ECT)

WIRING DIAGRAM

Refer to DTC P2195 on page 

DI–383

.

INSPECTION PROCEDURE

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 (AFS B2S1) and OS2 B1S2
(O2S B2S2)) 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.

+25 %

–12.5 %

More than 3.35 V

Less than 3.0 V

1

A/F Sensor (Sensor 1) 
Output Voltage

Injection volume

Output voltage

HO2 Sensor (Sensor 2) 
Output Voltage

Main Suspected 
Trouble Areas

OK

+25 %

–12.5 %

More than 3.35 V

Less than 3.0V

Injection volume

Output voltage

+25 %

–12.5 %

More than 0.55 V

Less than 0.4V

Injection volume

Output voltage

A/F sensor

+25 %

–12.5 %

More than 0.55 V

Less than 0.4V

Injection volume

Output voltage

+25 %

–12.5 %

Injection volume

Output voltage

NG

+25 %

–12.5 %

Injection volume

Output voltage

NG

+25 %

–12.5 %

Injection volume

Output voltage

NG

+25 %

–12.5 %

Injection volume

Output voltage

NG

OK

OK

OK

Almost 

no reaction



Almost 

no reaction

Almost 

no reaction

Almost 

no reaction

Case

2

3

4

A/F sensor circuit

A/F sensor heater

HO2 sensor

HO2 sensor circuit

HO2 sensor heater

(Air–fuel ratio extremely 
lean or rich)

Injector

Gas leakage from 
exhaust system

Fuel pressure

–

DIAGNOSTICS

ENGINE

DI–169

363

Standard:

Tester Display

(Sensor)

Injection Volumes

Status

Voltages

AFS B1S1 (AFS B2S1)

(A/F)

+25 %

Rich

Less than 3.0

AFS B1S1 (AFS B2S1)

(A/F)

–12.5 %

Lean

More than 3.35

O2S B1S2 (O2S B2S2)

(HO2)

+25 %

Rich

More than 0.55

O2S B1S2 (O2S B2S2)

(HO2)

–12.5 %

Lean

Less than 0.4

NOTICE:
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 A/F CONTROL procedure enables technicians to check and graph the voltage outputs
of both the A/F and HO2 sensors.

To display the graph, select the following menu items on the tester: DIAGNOSIS / ENHANCED OBD
II / ACTIVE TEST / A/F CONTROL / USER DATA / AFS B1S1 and O2S B1S2, and press the YES but-
ton and then the ENTER button followed by the F4 button.

DI–170

–

DIAGNOSTICS

ENGINE

364

HINT:

Read freeze frame data using a hand–held tester. Freeze frame data record 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.

A low A/F sensor voltage could be caused by a rich air–fuel mixture. Check for conditions that would
cause the engine to run rich.

A high A/F sensor voltage could be caused by a lean air–fuel mixture. Check for conditions that would
cause the engine to run lean.

1

Check any other DTCs output (in addition to DTC P0171, P0172, P0174 or P0175).

PREPARATION:
(a)

Connect a hand–held tester to the DLC3.

(b)

Turn the ignition switch to ON and turn the tester ON.

(c)

Select the following menu items: DIAGNOSIS / ENHANCED OBD II / DTC INFO / CURRENT CODES.

CHECK:
(a)

Read DTCs.
Result:

Display (DTC Output)

Proceed To

P0171, P0172, P0174 or P0175

A

P0171, P0172, P0174 or P0175 and other DTCs

B

HINT:
If any DTCs other than P0171, P0172, P0174 or P0175 are output, troubleshoot those DTCs first.

B

Go to DTC chart (See page 

DI–58

).

A

2

Check connection of PCV piping.

OK:

PCV hose is connected correctly and is not damaged.

NG

Repair or replace PCV piping.

OK