Subaru Legacy III (2000-2003 year). Manual - part 486

EN(H4DOSTC)-114

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Q: DTC P0172 — SYSTEM TOO RICH (BANK 1) —

• TROUBLE SYMPTOM:

• Erroneous idling
• Engine stalls.
• Poor driving performance

CAUTION:
After repair or replacement of faulty parts, conduct Clear Memory Mode <Ref. to EN(H4DOSTC)-35, 
OPERATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4DOSTC)-33, Inspection 
Mode.> .

Step

Value

Yes

No

1

CHECK EXHAUST SYSTEM.
Are there holes or loose bolts on exhaust sys-
tem?

There is no problem.

Go to step 2.

Repair the exhaust 
system.

2

CHECK AIR INTAKE SYSTEM.
Are there holes, loose bolts or disconnection of 
hose on air intake system?

There is no problem.

Go to step 3.

Repair the air 
intake system.

3

CHECK PCV VALVE.
Is PCV valve clogged?

PCV valve is not clogged.

Go to step 4.

Replace PCV 
valve.

4

CHECK FUEL PRESSURE.

Warning:

•Place “NO FIRE” signs near the working 
area.
•Be careful not to spill fuel on the floor.

1) Release the fuel pressure.

(1) Disconnect the connector from fuel 

pump relay.

(2) Start the engine and run it until it stalls.
(3) After the engine stalls, crank it for 5 

more seconds.

(4) Turn the ignition switch to OFF.

2) Connect the connector to fuel pump relay.
3) Disconnect the fuel delivery hose from fuel 

filter, and connect fuel pressure gauge.

4) Install the fuel filler cap.
5) Start the engine and idle while gear position 

is neutral.

6) Measure the fuel pressure while discon-

necting pressure regulator vacuum hose 
from intake manifold.
Is the measured value within the specified 
value?

Warning:

Before removing the fuel pressure gauge,
release fuel pressure.

NOTE:

If the fuel pressure does not increase, squeeze
fuel return hose 2 to 3 times, then measure fuel
pressure again.

284 — 314 kPa (2.9 — 3.2 kg/

cm

2

, 41 — 46 psi)

Go to step 5.

Repair the follow-
ing items.
Fuel presure too 
high
• Clogged fuel 
return line or bent 
hose
Fuel pressure too 
low
• Improper fuel 
pump discharge
• Clogged fuel sup-
ply line

EN(H4DOSTC)-115

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

5

CHECK FUEL PRESSURE.
After connecting pressure regulator vacuum 
hose, measure fuel pressure.
Is the measured value within the specified 
value?

Warning:

Before removing the fuel pressure gauge,
release fuel pressure.

NOTE:

•If the fuel pressure does not increase, 
squeeze fuel return hose 2 to 3 times, then 
measure fuel pressure again.
•If out of specification as measured at this 
step, check or replace the pressure regulator 
and pressure regulator vacuum hose.

206 — 235 kPa (2.1 — 2.4 kg/

cm

2

, 30 — 34 psi)

Go to step 6.

Repair the follow-
ing items.
Fuel pressure too 
high
• Faulty pressure 
regulator
• Clogged fuel 
return line or bent 
hose
Fuel pressure too 
low
• Faulty pressure 
regulator
• Improper fuel 
pump discharge
• Clogged fuel sup-
ply line

6

CHECK ENGINE COOLANT TEMPERATURE 
SENSOR.
1) Start the engine and warm-up completely.
2) Read the data of engine coolant tempera-

ture sensor signal using Subaru Select 
Monitor.
Does the measured value exceed the spec-
ified value?

NOTE:

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

60

°

C (140

°

F)

Go to step 7.

Replace the 
engine coolant 
temperature sen-
sor. <Ref. to 
FU(H4DOSTC)-
28, Engine Cool-
ant Temperature 
Sensor.>

7

CHECK INTAKE MANIFOLD PRESSURE 
SENSOR.
1) Start the engine and warm-up engine until 

coolant temperature is greater than 60

°

C 

(140

°

F).

2) Place the shift lever in neutral position.
3) Turn the A/C switch to OFF.
4) Turn all accessory switches to OFF.
5) Read the data of intake manifold pressure 

sensor signal using Subaru Select Monitor.

NOTE:

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

Is the measured value within the specified 
value?

Ignition ON 73.3 — 106.6 kPa 
(550 — 800 mmHg, 21.65 — 
31.50 inHg) Idling 24.0 — 41.3 
kPa (180 — 310 mmHg,    7.09 
— 12.20 inHg)

Go to step 8.

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

Step

Value

Yes

No

EN(H4DOSTC)-116

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

8

CHECK INTAKE AIR TEMPERATURE SEN-
SOR.
1) Start the engine and warm-up engine until 

coolant temperature is greater than 60

°

C 

(140

°

F).

2) Place the shift lever in neutral position.
3) Turn the A/C switch to OFF.
4) Turn all accessory switches to OFF.
5) Open the front hood.
6) Measure the ambient temperature.
7) Read the data of intake manifold pressure 

sensor signal using Subaru Select Monitor.
Is the value obtained when ambient tem-
perature is subtracted from intake air tem-
perature within the specified range?

NOTE:

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

−

10

°

C (14

°

F) — 50

°

C (122

°

F) Contact SUBARU 

distributor service.

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

Check 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)-117

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

MEMO: