Lexus SC300 / Lexus SC400. Manual - part 680

Diag. Code  71

EGR System Malfunction (Only for USA
spec.)

CIRCUIT DESCRIPTION

The EGR system recirculates exhaust gas, which is controlled to the proper quantity to suit the driving condi-
tions, into the intake air mixture to slow down combustion, reduce the combustion temperature and reduce NOx
emissions.
The lift amount of the EGR valve is controlled by a step motor operated by the ECU. The ECU operates the motor
only a preprogrammed amount in response to the engine operating conditions (engine rpm, intake air volume
to adjust the EGR volume to the target valve.)
Under the following conditions, EGR is cut to maintain driveability.

•

Coolant temp. below 535C (127.45F)

•

During deceleration (throttle valve
closed)

•

Light engine load (amount of intake air
very small)

•

Engine speed over 4,000 rpm

•

Engine idling

•

Neutral start switch on

Code No.

Diagnostic Code Detecting Condition

Trouble Area

(1) Open or short in EGR step motor circuit for 1 sec.

or more.

Open or short in EGR step motor circuit

ECU

71

(2)  EGR gas temp. is 65

°

C (149

°

F) or below for 

1 ~ 4 min. under conditions (a) and(b).

(2 trip detection logic)*

(a) Coolant temp.: 65

°

C (149

°

F) or more.

(b) EGR operation possible (EX. ECT in 3rd speed,

A/C ON, 60 mph (96 km/h), Flat road).

Open in EGR gas temp. sensor circuit

ECU

*: See page 

TR–21

.

TR–108

–

ENGINE TROUBLESHOOTING

CIRCUIT DESCRIPTION (Cont’d)

DIAGNOSIS CODE DETECTION DRIVING PATTERN

Purpose of the driving pattern.
(a)  To simulate diag. code detecting condition after diag. code is recorded.
(b)  To check that the malfunction is corrected when the repair is completed confirming that diag. code is no

longer detected.

Initiate test mode (See page 

TR–12

).



Start engine and warm up.



After engine is warmed up, let it idle for 3 min.



With the A/C ON and transmission in D range (O/D OFF), drive at 40–70 mph for 4 min.

HINT: 

If a malfunction exists, the ”CHECK” engine warning light will light up at 1–4 min. of driving at
40–70 mph (64–112 km/h).

NOTICE: 

If the conditions in this test are not strictly followed, detection of the malfunction will
not be possible.

Malfunction: Open in EGR gas temp. sensor circuit

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–109

DIAGNOSTIC CHART

WIRING DIAGRAM

Check voltage of terminal EGR 1 ~ 4.

Check voltage of EGR Step motor power
source.

Check resistance of EGR step motor.

Check for open and short in harness and
connector between EGR step motor and
ECU.

Check and replace ECU.

Check operation for EGR step motor.

Check resistance of EGR gas temp. sensor.

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

Check and replace ECU.

Replace EGR gas temp. sensor.

Replace EGR step motor.

Replace EGR step motor.

Repair or replace harness or
connector.

Repair or replace harness or
connector.

Check for ECU power source
circuit.

TR–110

–

ENGINE TROUBLESHOOTING

NG

OK

OK

NG

INSPECTION PROCEDURE

1

Check voltage between terminals EGR 1 ~ 4 of engine & ECT ECU connector and body ground.

C

OK

P

(2) Connect the Check Harness A.

(See page 

TR–30

).

(2) Turn ignition switch on.

Measure voltage between terminals EGR 1 ~ 4 of en-
gine & ECT ECU connector and body ground.

Voltage:

10 – 14 V

Go to step 5.

2

Check voltage between terminals B1 and B2 of EGR step motor connector and body ground.

C

OK

P

(2) Disconnect EGR step motor connector.

(2) Turn ignition switch on.

Measure voltage between terminals B1 and B2 of EGR
step motor connector and body ground.

Voltage:

10 – 14 V

Check for ECU power source circuit 
(See page 

TR–124

).

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–111