Lexus SC300 / Lexus SC400. Manual - part 870

OK

NG

OK

NG

NG

YES

7

Check each item found to be a possible cause of problem.

Check each circuit found to be a possible cause of trouble according to the results of the check in 

. The

numbers in the table below show the order in which the checks should be performed.

Main oxygen sensor

signal from either

side continues at 5 V.

Main oxygen sensor

signals from both

sides continue at 5 V.

Main oxygen sensor

signals from both

sides are normal.

Possible Cause

See page

1

7

Injector circuit

TR–128

3

Misfire

IG–6

2

4

Valve timing

EM–22

1

Air leakage

FI–7

1

2

Fuel system

TR–110

4

8

Characteristics deviation
in air flow meter.

TR–82

2

5

Characteristics deviation
in water temp. sensor.

TR–64

3

6

Characteristics deviation
in intake air temp. sensor.

TR–68

Repair or replace.

8

Check compression (See page 

EM–22

).

Repair or replace.

9

Does malfunction disappear when a good main oxygen sensor is installed?

Replace main oxygen sensor.

Check and replace engine (& ECT) ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–75

Diag. Code  24

Intake Air Temp. Sensor Circuit

CIRCUIT DESCRIPTION

The intake air temp. sensor is built into the air flow meter and senses the intake air temperature.
The structure of the sensor and connection to the ECU is the same as in the water temp. sensor shown on page

TR–68

.

If the ECU records the diagnostic code ”24”, it operates the fail safe function, keeping the intake air temperature
at a constant 20

°

C (68

°

F).

Code No.

Diagnostic Code Detecting Condition

Trouble Area

24

Open or short in intake air temp. sensor circuit
for 0.5 sec. or more.

Open or short in intake air temp. sensor circuit.

Intake air temp. sensor

ECU

DIAGNOSTIC CHART

DIAGNOSTIC CHART

HINT: If diagnostic codes ”22” (water temperature sensor circuit), ”24” (intake air temperature sensor

circuit) and ”41” (throttle position sensor circuit) are output simultaneously, E2 (sensor ground) may
be open.

WIRING DIAGRAM

Check and replace ECU.

Replace water temp. sensor.

Check for momentary interruption

 (See page 

TR–23

).

Repair or replace harness or
connector.

Check voltage of sensor.

Check resistance of sensor.

Check for open and short in harness and
connector between ECU and water temp. sensor.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–72

NG

OK

OK

NG

OK

NG

INSPECTION PROCEDURE

1

Check voltage between terminals THA and E2 of engine & ECT ECU connector.

C

OK

P

(2) Connect the Check Harness A.

(See page TR–30)

(2) Turn ignition switch on.

Measure voltage between terminals THA and
E2 and engine & ECT ECU connector.

Check for momentary Intertuption
(See page 

TR–23

).

2

Check intake air temp. sensor.

C

OK

P

Disconnect the air flow meter connector.

Measure resistance between terminals 1 and 2 of air
flow meter connector.

Resistance is within Acceptable Zone on chart.

Replace intake air temp. sensor (Replace air flow meter).

3

Check for open and short in harness and connector between engine & ECT ECU and intake air

temp. sensor (See page 

IN–27

).

Repair or replace harness or connector.

Check and replace engine & ECT ECU.

TR–73

–

ENGINE TROUBLESHOOTING

Circuit Inspection

Diag Code

25

Air–Fuel Ratio Lean Malfunction

Diag. Code

26

Air–Fuel Ratio Rich Malfunction

CIRCUIT DESCRIPTION

Refer to page 

TR–62

 for the circuit description.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

(1)

Main oxygen sensor voltage is 0.45 V or less (lean)
for 90 sec. under conditions (a) and (b).
(2 trip detection logic)*
(a) Coolant temp.: 60

°

C (140

°

F) or more.

(b) Engine speed: 1,500 rpm or more.

Open or short in main oxygen sensor circuit

Main oxygen sensor

Ignition system

Water temp. sensor

25

⋅

26

(2)  Main oxygen sensor voltage is alternating

above and below 0.45 V at 5 times par
second or more under conditions (a) and (b).
(2 trip detection logic)*
(a) Engine speed: Idling
(b) Coolant temp.: Between 60

°

C (140

°

F)

and 95

°

C (203

°

F)

Open or short in injector circuit.

Fuel line pressure (injector blockage)

Air flow meter (air intake)

Engine ground bolt loose

Foreign object caught in valve

(3) Difference of air–fuel ratio feedback compensation

value between right and left banks is more than 10
percentage for 30 sec. or more under conditions (a)
and (b).
(2 trip detection logic)*
(a) Engine speed : 2,000 rpm or more.
(b) Coolant temp.: Between 60

°

C (140

°

F)

and 95

°

C (203

°

F)

Fuel line pressure (injector leak, blockage)

Mechanical system malfunction

 (skipping teeth of timing belt)

Ignition system

*: See page 

TR–21

.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–74