Subaru Legacy III (2000-2003 year). Service manual — part 477

EN(H4DOSTC)-78

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

E: DTC P0102 — MASS OR VOLUME AIR FLOW CIRCUIT LOW INPUT —

• TROUBLE SYMPTOM:

• Erroneous idling
• Engine stalls.
• Poor driving performance

• WIRING DIAGRAM:

EN-00951

B122

B3

B135

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

BATTERY

E

B122

1

B3

MASS AIR FLOW AND
INTAKE AIR TEMPERATURE

ECM

B135

B :

B84

E :

SBF-5

1 2 3 4 5

3

4

1

2

5

6

1 2 3 4 5 6

B84

B47

2
6

2

4

3

5

E7

B27

E13

B19

E8

1 2 3

8 9 10

4

11 12

13 14 15

16

5 6

7

17

MAIN RELAY

B47

1

2

3

5

4

6

EN(H4DOSTC)-79

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Step

Value

Yes

No

1

CONNECT SUBARU SELECT MONITOR OR
THE OBD-II GENERAL SCAN TOOL, AND
READ DATA.
1) Turn the ignition switch to OFF.
2) Connect the Subaru Select Monitor or the

OBD-II general scan tool to data link con-
nector.

3) Turn the ignition switch to ON and Subaru

Select Monitor or the OBD-II general scan
tool switch to ON.

4) Start the engine and run it at idle.
5) Read the data of mass air flow sensor sig-

nal using Subaru Select Monitor.
Is the measured value within the specified
value?

NOTE:

For detailed operation procedure, refer to the
“READ CURRENT DATA FOR ENGINE”. <Ref.
to EN(H4DOSTC)-26, Subaru Select Monitor.>

1.3 g/sec (0.172 lb/min) — 240
g/sec (32 lb/min) or 0.3 — 4.58
V

Even if MI lights
up, the circuit has
returned to a nor-
mal condition at
this time. A tempo-
rary poor contact
of the connector or
harness may be
the cause. Repair
harness or con-
nector in the mass
air flow sensor.

NOTE:
In this case, repair
the following:
• Open or ground
short circuit in 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 FOR ECM.
Measure the voltage between ECM connector
and chassis ground while engine is idling.

Connector & terminal

(B84) No. 13 (+) — Chassis ground (

−−−−

):

Is the measured value less than the specified
value?

0.2 V

Go to step 4.

Go to step 3.

3

CHECK INPUT SIGNAL FOR ECM (USING
SUBARU SELECT MONITOR).
Measure the voltage between ECM connector
and chassis ground while engine is idling.
Does the measured value change by shaking
harness and connector of ECM while monitor-
ing the value with Subaru Select Monitor?

The value changes.

Repair the poor
contact in ECM
connector.

Contact SUBARU
distributor service.

NOTE:
Inspection by DTM
is required, be-
cause probable
cause is deteriora-
tion of multiple
parts.

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 (

−−−−

):

Does the measured value exceed the spec-
ified value?

10V

Go to step 5.

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

EN(H4DOSTC)-80

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

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
connector.

Connector & terminal

(B84) No. 13 — (B3) No. 3:
(B135) No. 27 — (B3) No. 4:
(B135) No. 19 — (B3) No. 5:

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

1

Ω

Go to step 6.

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

6

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

Connector & terminal

(B84) No. 13 — Chassis ground:
(B135) No. 27 — Chassis ground:
(B135) No. 19 — Chassis ground:

Does the measured value exceed the specified
value?

1M

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?

There is poor contact.

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

Replace the mass
air flow and intake
air temperature
sensor. <Ref. to
FU(H4DOSTC)-
33, Mass Air Flow
and Intake Air
Temperature Sen-
sor.>

Step

Value

Yes

No

EN(H4DOSTC)-81

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

MEMO: