Subaru Legacy (2005 year). Service manual - part 335

EN(H4DOTC)(diag)-109

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

Step

Check

Yes

No

1

CHECK ANY OTHER DTC ON DISPLAY.

Is any other DTC displayed?

Check DTC using 
“List of Diagnostic 
Trouble Code 
(DTC)”. <Ref. to 
EN(H4DOTC)(diag
)-62, List of Diag-
nostic Trouble 
Code (DTC).>

Go to step 2.

2

CHECK REAR OXYGEN SENSOR DATA.
1) Warm-up the engine until engine coolant 
temperature is above 70

°C (158°F), and keep 

the engine speed at 2,000 to 3,000 rpm for two 
minutes.
2) Read the data of rear oxygen sensor signal 
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(H4DOTC)(diag)-24, Subaru Select Mon-
itor.>
• General scan tool
For detailed operation procedure, refer to the 
general scan tool operation manual.

Does the value fluctuate?

Go to step 6.

Go to step 3.

3

CHECK REAR OXYGEN SENSOR DATA.
Read the data of rear oxygen sensor signal 
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(H4DOTC)(diag)-24, Subaru Select Mon-
itor.>
• General scan tool
For detailed operation procedure, refer to the 
general scan tool operation manual.

Is the voltage 0.2 — 0.4 V?

Go to step 4.

Replace the rear 
oxygen sensor. 
<Ref. to 
FU(H4DOTC)-33, 
Rear Oxygen Sen-
sor.>

4

CHECK HARNESS BETWEEN ECM AND 
REAR OXYGEN SENSOR CONNECTOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from ECM and 
rear oxygen sensor.
3) Measure the resistance of harness 
between ECM and rear oxygen sensor con-
nector.

Connector & terminal

(B137) No. 25 — (T6) No. 4:

Is the resistance more than 3 
Ω?

Repair the open 
circuit of harness 
between ECM and 
rear oxygen sen-
sor connector.

Go to step 5.

EN(H4DOTC)(diag)-110

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

T: DTC P0171 SYSTEM TOO LEAN (BANK 1)

NOTE:
For the diagnostic procedure, refer to DTC P0172. <Ref. to EN(H4DOTC)(diag)-111, DTC P0172 SYSTEM
TOO RICH (BANK 1), Diagnostic Procedure with Diagnostic Trouble Code (DTC).>

5

CHECK HARNESS BETWEEN REAR OXY-
GEN SENSOR AND ECM CONNECTOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from rear oxygen 
sensor.
3) Turn the ignition switch to ON.
4) Measure the voltage between rear oxygen 
sensor harness connector and chassis ground.

Connector & terminal

(T6) No. 4 (+) — Chassis ground (

−

):

Is the voltage more than 0.2 V? Replace the rear 

oxygen sensor. 
<Ref. to 
FU(H4DOTC)-33, 
Rear Oxygen Sen-
sor.>

Repair the har-
ness and connec-
tor.

NOTE:
In this case, repair
the following:
• Open circuit of 
harness between 
rear oxygen sen-
sor and ECM con-
nector
• Poor contact in 
rear oxygen sen-
sor connector
• Poor contact in 
ECM connector

6

CHECK EXHAUST SYSTEM.
Check exhaust system parts.

NOTE:

Check the following items.
• Loose part and incomplete installation of 
exhaust system
• Damage (crack, hole etc.) of parts
• Looseness and ill fitting of parts between 
front oxygen (A/F) sensor and rear oxygen 
sensor

Is there any fault in exhaust 
system?

Repair or replace 
faulty parts.

Replace the rear 
oxygen sensor. 
<Ref. to 
FU(H4DOTC)-33, 
Rear Oxygen Sen-
sor.>

Step

Check

Yes

No

EN(H4DOTC)(diag)-111

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

U: DTC P0172 SYSTEM TOO RICH (BANK 1)

DTC DETECTING CONDITION:
Detects when malfunction occurs in 2 continuous driving cycles.
TROUBLE SYMPTOM:
• Erroneous idling
• Engine stalls.
• Poor driving performance

CAUTION:
After repair or replacement of faulty parts, perform Clear Memory Mode <Ref. to EN(H4DOTC)(diag)-
37, OPERATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4DOTC)(diag)-30, PROCE-
DURE, Inspection Mode.>.

Step

Check

Yes

No

1

CHECK EXHAUST SYSTEM.

Are there holes or loose bolts 
on exhaust system?

Repair the exhaust 
system.

Go to step 2.

2

CHECK AIR INTAKE SYSTEM.

Are there holes, loose bolts or 
disconnection of hoses on air 
intake system?

Repair the air 
intake system.

Go to step 3.

3

CHECK FUEL PRESSURE.

Warning:

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

1) Release the fuel pressure.

(1) Disconnect the connector from fuel 
pump relay.
(2) Start the engine and run until it stalls.
(3) After the engine stalls, crank it for five 
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 the fuel pressure gauge.
4) Install the fuel filler cap.
5) Start the engine and idle while gear posi-
tion is neutral.
6) Measure the fuel pressure while discon-
necting pressure regulator vacuum hose from 
intake manifold.

Warning:

Release fuel pressure before removing the
fuel pressure gauge.

NOTE:

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

Is the measured value 284 — 

314 kPa (2.9 — 3.2 kgf/cm

2

, 41 

— 46 psi)?

Go to step 4.

Repair the follow-
ing items.
Fuel pressure is 
too high:

• Clogged fuel 
return line or 
bent hose

Fuel pressure is 
too low:

• Improper fuel 
pump discharge
• Clogged fuel 
supply line

EN(H4DOTC)(diag)-112

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

4

CHECK FUEL PRESSURE.
After connecting the pressure regulator vac-
uum hose, measure fuel pressure.

Warning:

Release fuel pressure before removing the
fuel pressure gauge.

NOTE:

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

Is the measured value 206 — 

235 kPa (2.1 — 2.4 kgf/cm

2

, 30 

— 34 psi)?

Go to step 5.

Repair the follow-
ing items.
Fuel pressure is 
too high:

• Faulty pres-
sure regulator
• Clogged fuel 
return line or 
bent hose

Fuel pressure is 
too low:

• Faulty pres-
sure regulator
• Improper fuel 
pump discharge
• Clogged fuel 
supply line

5

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 
or general scan tool.

NOTE:

• Subaru Select Monitor
For detailed operation procedure, refer to 
“READ CURRENT DATA FOR ENGINE”. <Ref. 
to EN(H4DOTC)(diag)-24, Subaru Select Mon-
itor.>
• General scan tool
For detailed operation procedure, refer to the 
general scan tool operation manual.

Is the temperature more than 
60

°C (140°F)?

Go to step 6.

Replace the 
engine coolant 
temperature sen-
sor. <Ref. to 
FU(H4DOTC)-22, 
Engine Coolant 
Temperature Sen-
sor.>

6

CHECK MASS AIR FLOW AND INTAKE AIR 
TEMPERATURE SENSOR.
1) Start and warm-up the engine until engine 
coolant temperature is above 60

°C (140°F).

2) Place the shift lever in neutral position.
3) Turn the A/C switch to OFF.
4) Turn all the accessory switches to OFF.
5) Read the data of mass air flow and intake 
air temperature sensor signal 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(H4DOTC)(diag)-24, Subaru Select Mon-
itor.>
• General scan tool
For detailed operation procedure, refer to the 
general scan tool operation manual.

Is the measured value within 
the following? 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 7.

Replace the mass 
air flow and intake 
air temperature 
sensor. <Ref. to 
FU(H4DOTC)-27, 
Mass Air Flow and 
Intake Air Temper-
ature Sensor.>

Step

Check

Yes

No