Subaru Legacy III (2000-2003 year). Manual - part 219

EN(H4SOw/oOBD)-66

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

7

CHECK ECM.
1) Connect all connectors.
2) Erase the memory. <Ref. to EN(H4SOw/

oOBD)-28, OPERATION, Clear Memory 
Mode.>

3) Perform inspection mode. <Ref. to 

EN(H4SOw/oOBD)-26, OPERATION, 
Inspection Mode.>

4) Read out the trouble code. <Ref. to 

EN(H4SOw/oOBD)-24, OPERATION, 
Read Diagnostic Trouble Code (DTC).>
Is the same trouble code (DTC) as in the 
current diagnosis still being output?

DTC is indicated.

Replace ECM.

Go to step 8.

8

CHECK ANY OTHER TROUBLE CODES AP-
PEARANCE.
Are other trouble codes (DTC) being output?

DTC is indicated.

Proceed with the 
diagnosis corre-
sponding to the 
trouble code.

A temporary poor 
contact.

Step

Value

Yes

No

EN(H4SOw/oOBD)-67

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

D: DTC 21 ENGINE COOLANT TEMPERATURE SENSOR

• DIAGNOSIS:

• The engine coolant temperature sensor signal is abnormal.
• The harness connector between ECM and engine coolant temperature sensor is in short or open.

• TROUBLE SYMPTOM:

• Hard to start
• Erroneous idling
• Poor driving performance

CAUTION:
After repair or replacement of faulty parts, conduct CLEAR MEMORY and INSPECTION MODES. <Ref. 
to EN(H4SOw/oOBD)-28, OPERATION, Clear Memory Mode.>  and <Ref. to EN(H4SOw/oOBD)-26, OP-
ERATION, Inspection Mode.>
• WIRING DIAGRAM:

EN-01052

16

14

9

13

B136 ECM

2

1

E2

B21

E8

ENGINE

COOLANT

TEMPERATURE

SENSOR

E8

6

4

B83

5

4

6 7

8

2

1

9

3

10

22

23

11 12 13 14 15

24 25

26 27

16 17 18

28 29

19 20

21

30

B21

1 2

3 4

5

6 7

8

9 10

11 12

13

14 15

16

17 18

19 20

B136

B83

1 2

3

1 2 3 4 5 6 7 8 9 10 11 12

EN(H4SOw/oOBD)-68

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Step

Value

Yes

No

1

CHECK HARNESS BETWEEN ENGINE 
COOLANT TEMPERATURE SENSOR AND 
ECM CONNECTOR.
1) Turn ignition switch to OFF.
2) Remove generator.
3) Disconnect connector from engine coolant 

temperature sensor.

4) Measure voltage between engine coolant 

temperature sensor connector and engine 
ground.

Connector & terminal

(E8) No. 1 (+) — Engine ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

10 V

Repair battery 
short circuit in har-
ness between 
ECM and engine 
coolant tempera-
ture sensor con-
nector.

Go to step 2.

2

CHECK HARNESS BETWEEN ENGINE 
COOLANT TEMPERATURE SENSOR AND 
ECM CONNECTOR.
1) Turn ignition switch to ON.
2) Measure voltage between engine coolant 

temperature sensor connector and engine 
ground.

Connector & terminal

(E8) No. 1 (+) — Engine ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

10 V

Repair battery 
short circuit in har-
ness between 
ECM and engine 
coolant tempera-
ture sensor con-
nector.

Go to step 3.

3

CHECK HARNESS BETWEEN ENGINE 
COOLANT TEMPERATURE SENSOR AND 
ECM CONNECTOR.
Measure voltage between engine coolant tem-
perature sensor connector and engine ground.

Connector & terminal

(E8) No. 1 (+) — Engine ground (

−−−−

):

Does the measured value exceed the specified 
value?

4 V

Go to step 4.

Repair harness 
and connector.

NOTE:
In this case, repair
the following:
• Open circuit in 
harness between 
ECM and engine 
coolant tempera-
ture sensor con-
nector
• Poor contact in 
engine coolant 
temperature sen-
sor connector
• Poor contact in 
ECM connector
• Poor contact in 
coupling connector 
(B21)

EN(H4SOw/oOBD)-69

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

4

CHECK HARNESS BETWEEN ENGINE 
COOLANT TEMPERATURE SENSOR AND 
ECM CONNECTOR.
1) Turn ignition switch to OFF.
2) Measure resistance of harness between 

engine coolant temperature sensor connec-
tor and engine ground.

Connector & terminal

(E8) No. 2 — Engine ground:

Is the measured value less than the speci-
fied value?

5 

Ω

Go to step 5.

Repair harness 
and connector.

NOTE:
In this case, repair
the following:
• Open circuit in 
harness between 
ECM and engine 
coolant tempera-
ture sensor con-
nector
• Poor contact in 
engine coolant 
temperature sen-
sor connector
• Poor contact in 
ECM connector
• Poor contact in 
coupling connector 
(B21)
• Poor contact in 
sensor ground 
joint connector 
(B83)

5

CHECK ENGINE COOLANT TEMPERATURE 
SENSOR.
1) Remove engine coolant temperature sen-

sor.

2) Immerse sensor in approx. 20

°

C (68

°

F) 

water.

NOTE:

Prevent water from entering into terminal part
of the sensor.

3) Measure resistance between engine cool-

ant temperature sensor terminals.

Terminals

No. 1 — No. 2:

Is the measured value within the specified 
value?

2 — 3 k

Ω

Go to step 6.

Replace engine 
coolant tempera-
ture sensor.

6

CHECK ENGINE COOLANT TEMPERATURE 
SENSOR.
1) Raise temperature up to approx. 80

°

C 

(176

°

F).

CAUTION:

Be careful not to burn yourself.

2) Measure resistance between engine cool-

ant temperature sensor terminals.

Terminals

No. 1 — No. 2:

Is the measured value within the specified 
value?

0.35 — 0.4 k

Ω

Go to step 7.

Replace engine 
coolant tempera-
ture sensor.

Step

Value

Yes

No