Lexus SC300 / Lexus SC400. Manual - part 868

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.

Repair or replace harness or con-
nector.

Replace water temp. sensor.

Check voltage of sensor.

Check resistance of sensor.

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

Check for momentary interruption

 (See page 

TR–23

).

TR–69

–

ENGINE TROUBLESHOOTING

Circuit Inspection

NG

OK

OK

NG

OK

NG

INSPECTION PROCEDURE

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.

1

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

Check for momentary interruption
(See Page 

TR–23

).

2

Check water temp. sensor.

C

OK

P

Disconnect the water temp. sensor connector.
(See page FI–99).

Measure resistance between terminals.

Resistance is within Acceptable Zone on chart.

Replace water temp. sensor.

3

Check for open and short in harness and connector between engine & ECT ECU and water temp.

sensor (See page 

IN–27

).

Repair or replace harness or connector.

Check and replace engine & ECT ECU.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–70

–MEMO–

TR–71

ENGINE TROUBLESHOOTING

Diag Code 25 26

Air–Fuel Ratio Lean Malfunction

Diag. Code 25, 26

Air–Fuel Ratio Rich Malfunction

CIRCUIT DESCRIPTION

Refer to page 

TR–62

 for the circuit description.

Code No.

Diagnostic Code Detecting System

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)

*2

(a) Coolant temp.: 70

°

C (158

°

F)

or more.

(b) Engine speed: 1,500 rpm or more.



Open or short in main oxygen sensor
circuit



Main oxygen sensor



Ignition system



ECU

25

(2)

*1

 Difference of air–fuel ratio feedback

compensation value between front (No.
1  3 cylinders) and rear (No. 4  6
cylinders) is more than 15 percentage
for 20 sec. or more under conditions (a)
and (b).
(2 trip detection logic)

*2

(a) Engine speed: 2,000 rpm or more.
(b) Coolant temp.: Between 60

°

C

(140

°

F) and 95

°

C (203

°

F).



Open and short in injector circuit.



Fuel line pressure (injector leak, blockage)



Mechanical system malfunction (skipping
teeth of timing belt)



Ignition system



Compression pressure (foreign object
caught in valve)



Air flow meter (air intake)



ECU

(3)

*1

 Engine speed varies by more than 15

rpm over the preceding crank angle period
during a period of 20 sec. or more under
conditions (a) and (b).
(2 trip detection logic)

*2

(a) Engine speed: Idling
(b) Coolant temp.: Between 60

°

C

(140

°

F) and 95

°

C (203

°

F).



Open and short in injector circuit.



Fuel line pressure (injector leak, blockage)



Mechanical system malfunction (skipping
teeth of timing belt)



Ignition system



Compression pressure (foreign object
caught in valve)



Air flow meter (air intake)



ECU

26

*1

(1) Difference of air–fuel ratio feedback

compensation value between front (No. 1
3 cylinders) and rear (No.4  6 cylinders) is
more than 15 percentage for 20 sec. or
more under conditions (a) and (b).
(2 trip detection logic)

*2

(a) Engine speed: 2,000 rpm or more.
(b) Coolant temp.: Between 60

°

C

(140

°

F) and 95

°

C (203

°

F).



Open and short in injector circuit.



Fuel line pressure (injector leak, blockage)



Mechanical system malfunction (skipping
teeth of timing belt)



Ignition system



Compression pressure (foreign object
caught in valve)



Air flow meter (air intake)



ECU

26

1

(2) Engine speed varies by more than 15

rpm over the preceding crank angle peri-
od during a period of 20 sec. or more un-
der conditions (a) and (b).
(2 trip detection logic)

*2

(a) Engine speed: Idling
(b) Coolant temp.: Between 60

°

C

(140

°

F) and 95

°

C (203

°

F).



Open and short in injector circuit.



Fuel line pressure (injector leak, blockage)



Mechanical system malfunction (skipping
teeth of timing belt)



Ignition system



Compression pressure (foreign object
caught in valve



Air flow meter (air intake)



ECU

*1: Only for USA specification vehicles.
*2: See page 

TR–25

.

TR–70

–

ENGINE TROUBLESHOOTING

Circuit Inspection