Lexus ES300 (1997 year). Manual - part 319

–

DIAGNOSTICS

ENGINE

DI–53

205

1997 LEXUS ES300   (RM511U)

3

Check output voltage of heated oxygen sensor (bank 1 sensor 2).

PREPARATION:
(a)

Connect the OBD II scan tool or LEXUS hand–held tester to the DLC3.

(b)

Warm up engine to normal operating temp.

CHECK:
Read voltage output of heated oxygen sensor (bank 1 sensor 2) when engine suddenly raced.
HINT:Perform quick racing to 4,000 rpm 3 min. using accelerator pedal.
OK:

Heated oxygen sensor output voltage:  Alternates from 0.4 V or less to 0.5 V or more.

OK

 Check that each connector is properly
 connected.

NG

Replace heated oxygen sensor
 (bank 1 sensor 2).

DI–54

–

DIAGNOSTICS

ENGINE

206

1997 LEXUS ES300   (RM511U)

DTC

P0171

System too Lean (Fuel Trim)

DTC

P0172

System too Rich (Fuel Trim)

CIRCUIT DESCRIPTION

Fuel trim refers to the feedback compensation value compared against the basic injection time. Fuel trim
includes short–term fuel trim and long–term fuel trim.
Short–term fuel trim is the short–term fuel compensation used to maintain the air–fuel ratio at its ideal
theoretical value. The signal from the heated oxygen sensor indicates whether the air–fuel ratio is RICH or
LEAN compared to the ideal theoretical value, triggering a reduction in fuel volume if the air–fuel ratio is rich,
and an increase in fuel volume if it is lean.
Long–term fuel trim is overall fuel compensation carried out long–term to compensate for continual deviation
of the short–term fuel trim form the central value due to individual engine differences, wear over time and
changes in the usage environment.
If both the short–term fuel trim and long–term fuel trim are LEAN or RICH beyond a certain value, it is 
detected as a malfunction and the MIL lights up.

DTC No.

DTC Detecting Condition

Trouble Area

P0171

When the air fuel ratio feedback is stable after engine warming

up, the fuel trim is considerably in error on the RICH side

(2 trip detection logic)

Air intake (hose loose)

Fuel line pressure

Injector  blockage

Heated oxygen sensor (bank 1, 2 sensor 1) malfunction

Mass air flow meter

Engine coolant temp. sensor

P0172

When the air fuel ratio feedback is stable after engine warming

up, the fuel trim is considerably in error on the LEAN side

(2 trip detection logic)

Fuel line pressure

Injector leak, blockage

Heated oxygen sensor (bank 1, 2 sensor 1) malfunction

Mass air flow meter

Engine coolant temp. sensor

HINT:

When  DTC P0171 is recorded, the actual air–fuel ratio is on the LEAN side. When DTC P0172 is  re-
corded, the actual air–fuel ratio is on the RICH side.

If the vehicle runs out of fuel, the air–fuel ratio is LEAN and DTC P0171 is recorded. The MIL then
comes on.

If the total of the short–term fuel trim value and long–term fuel trim value is within 

±

 25 %, the system

is functioning normally.

INSPECTION PROCEDURE

1

Check air induction system (See page 

SF–1

).

NG

 Repair or replace.

OK

DI09N–01

–

DIAGNOSTICS

ENGINE

DI–55

207

1997 LEXUS ES300   (RM511U)

2

Check for heated oxygen sensors (bank 1, 2 sensor 1 ) data.

PREPARATION:
(a)

Connect the OBD II scan tool or LEXUS hand–held tester to the DLC3.

(b)

Warm up engine to normal operating temp.

CHECK:
Read the heated oxygen sensor (bank 1, 2 sensor 1) output voltage and short–term fuel trim.
HINT:Read the values for the same bank.
RESULT:

Pattern

Heated oxygen sensor output voltage

Short–term fuel trim

1

Lean condition (Changes at 0.55 V or less)

Changes at about + 20 %

2

RIch condition (Changes at 0.4 V or more)

Changes at about – 20 %

3

Except 1 and 2

3

 Check for heated oxygen sensor
(bank 1, 2 sensor 1) (See page 

DI–46

).

1, 2

3

Check fuel pressure (See page 

SF–6

).

NG

 Check and repair fuel pump, fuel pipe line and
 filter (See page 

SF–1

).

OK

4

Check injector injection (See page 

SF–20

).

NG

 Replace injector.

OK

DI–56

–

DIAGNOSTICS

ENGINE

208

1997 LEXUS ES300   (RM511U)

5

Check mass air flow meter and engine coolant temp. sensor 
(See page 

SF–29

, 

SF–57

).

NG

 Repair or replace.

OK

6

Check for spark and ignition (See page 

IG–1

).

NG

 Repair or replace.

OK

Check and replace ECM (See page 

IN–27

).