Subaru Legacy (2005 year). Manual - part 201

EN(H4SO 2.5)(diag)-89

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

Step

Check

Yes

No

1

CHECK OPTION CODE.

Is the option code EC, EK, EH, 
ER or K4?

Go to step 2.

Refer to EN(H4SO 
2.0) section. <Ref. 
to EN(H4SO 
2.0)(diag)-64, List 
of Diagnostic Trou-
ble Code (DTC).>

NOTE:
Fuel injection sys-
tem for KA and KS
model is the same
as 2.0 L model.

2

CHECK INPUT SIGNAL OF ECM.
Measure the voltage between ECM connector 
and chassis ground.

Connector & terminal

(B135) No. 2 (+) — Chassis ground (

−

):

Is the voltage more than 8 V?

Go to step 3.

Go to step 4.

3

CHECK CURRENT DATA.
1) Turn the ignition switch to OFF.
2) Repair the battery short circuit of harness 
between ECM and rear oxygen sensor con-
nector.
3) Turn the ignition switch to ON.
4) Read the data of rear oxygen sensor heater 
current using Subaru Select Monitor or general 
scan tool.

NOTE:

• Subaru Select Monitor
For detailed operation procedure, refer to 
“READ CURRENT DATA FOR ENGINE”. <Ref. 
to EN(H4SO 2.5)(diag)-25, Subaru Select 
Monitor.>
• General scan tool
For detailed operation procedure, refer to the 
general scan tool operation manual.

Is the current more than 7 A?

Replace the ECM. 
<Ref. to FU(H4SO 
2.5)-37, Engine 
Control Module 
(ECM).>

Finish the diagno-
sis.

4

CHECK POOR CONTACT.
Check poor contact in ECM connector.

Is there poor contact in ECM 
connector?

Repair the poor 
contact in ECM 
connector.

Finish the diagno-
sis.

EN(H4SO 2.5)(diag)-90

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

F: DTC P0102 MASS OR VOLUME AIR FLOW CIRCUIT LOW INPUT

DTC DETECTING CONDITION:
Immediately at fault recognition
TROUBLE SYMPTOM:
• Erroneous idling
• Engine stalls.
• Poor driving performance

CAUTION:
After repair or replacement of faulty parts, conduct Clear Memory Mode <Ref. to EN(H4SO 2.5)(diag)-
40, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4SO 2.5)(diag)-33, Inspection Mode.>.
WIRING DIAGRAM:

EN-03459

B3

BATTERY

E

1

B3

MASS AIR FLOW AND 

INTAKE AIR TEMPERATURE

SENSOR

ECM

B136

SBF-7

3

4

1

2

5

6

B47

2

4

3

5

31

13

23

35

32

MAIN RELAY

B47

1

2

6

3

5

1 2 3 4
5 6 7 8

B83

B136

5

6

7 8

2

1

9

4

3

10

24

22 23

25

11 12 13 14 15

26 27

28

16

17 18 19 20 21

33 34

29

32

30

31

35

*

: TERMINAL No. RANDOM ARRANGEMENT

B83

*

1 2 3 4 5

4

*

EN(H4SO 2.5)(diag)-91

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

Step

Check

Yes

No

1

CHECK CURRENT DATA.
1) Turn the ignition switch to OFF.
2) Connect the Subaru Select Monitor or gen-
eral scan tool to data link connector.
3) Turn the ignition switch to ON, and turn the 
power switch of Subaru Select Monitor or gen-
eral scan tool to ON.
4) Start the engine.
5) Read the voltage of mass air flow sensor 
using Subaru Select Monitor or general scan 
tool.

NOTE:

• Subaru Select Monitor
For detailed operation procedure, refer to 
“READ CURRENT DATA FOR ENGINE”. <Ref. 
to EN(H4SO 2.5)(diag)-25, Subaru Select 
Monitor.>
• General scan tool
For detailed operation procedure, refer to the 
general scan tool operation manual.

Is the voltage 0.2 — 4.7 V?

Even if the mal-
function indicator 
light illuminates, 
the circuit has 
returned to a nor-
mal condition at 
this time. A tempo-
rary poor contact 
in connector or 
harness may be 
the cause. Repair 
the harness or 
connector in mass 
air flow sensor.

NOTE:
In this case, repair
the following:
• Open or ground 
short circuit of har-
ness between 
mass air flow sen-
sor and ECM con-
nector
• Poor contact in 
mass air flow sen-
sor or ECM con-
nector

Go to step 2.

2

CHECK INPUT SIGNAL OF ECM.
Measure the voltage between ECM connector 
and chassis ground while engine is idling.

Connector & terminal

(B136) No. 23 (+) — Chassis ground (

−

):

Is the voltage more than 0.2 V? Go to step 4.

Go to step 3.

3

CHECK INPUT SIGNAL OF ECM (USING 
SUBARU SELECT MONITOR).
Measure the voltage between ECM connector 
and chassis ground while engine is idling.

Does the voltage change by 
shaking the harness and con-
nector of ECM while monitor-
ing the value with Subaru 
Select Monitor?

Repair the poor 
contact in ECM 
connector.

Replace the ECM. 
<Ref. to FU(H4SO 
2.5)-37, Engine 
Control Module 
(ECM).>

4

CHECK POWER SUPPLY TO MASS AIR 
FLOW SENSOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from mass air 
flow sensor.
3) Turn the ignition switch to ON.
4) Measure the voltage between mass air flow 
sensor connector and chassis ground.

Connector & terminal

(B3) No. 1 (+) — Chassis ground (

−

):

Is the voltage more than 5 V?

Go to step 5.

Repair the open 
circuit between 
mass air flow sen-
sor and main relay.

5

CHECK HARNESS BETWEEN ECM AND 
MASS AIR FLOW SENSOR CONNECTOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from ECM.
3) Measure the resistance of harness 
between ECM and mass air flow sensor con-
nector.

Connector & terminal

(B136) No. 23 — (B3) No. 3:
(B136) No. 31 — (B3) No. 2:
(B136) No. 35 — (B3) No. 5:

Is the resistance less than 1 
Ω?

Go to step 6.

Repair the open 
circuit between 
ECM and mass air 
flow sensor con-
nector.

EN(H4SO 2.5)(diag)-92

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

6

CHECK HARNESS BETWEEN ECM AND 
MASS AIR FLOW SENSOR CONNECTOR.
Measure the resistance of harness between 
ECM and chassis ground.

Connector & terminal

(B136) No. 23 — Chassis ground:
(B136) No. 31 — Chassis ground:
(B136) No. 35 — Chassis ground:

Is the resistance more than 1 
M

Ω?

Go to step 7.

Repair the ground 
short circuit 
between ECM and 
mass air flow sen-
sor connector.

7

CHECK POOR CONTACT.
Check poor contact in mass air flow sensor 
connector.

Is there poor contact in mass 
air flow sensor connector?

Repair the poor 
contact in mass air 
flow sensor con-
nector.

Replace the mass 
air flow and intake 
air temperature 
sensor. <Ref. to 
FU(H4SO 2.5)-26, 
Mass Air Flow and 
Intake Air Temper-
ature Sensor.>

Step

Check

Yes

No