Subaru Legacy 2004 Service Manual page 895

Diagnostic Procedure with Diagnostic Trouble Code (DTC)
ENGINE (DIAGNOSTICS)
Step
1 CHECK OPTION CODE.
2 CHECK ANY OTHER DTC ON DISPLAY.
X: DTC P0171 SYSTEM TOO LEAN (BANK 1)
Refer to DTC P0172 for diagnostic procedure. <Ref. to EN(H4SO 2.5)(diag)-137, DTC P0172 SYSTEM TOO
RICH (BANK 1), Diagnostic Procedure with Diagnostic Trouble Code (DTC).>
Check
Is the option code EC, EK or
K4?
Is any other DTC displayed?
EN(H4SO 2.5)(diag)-136
Yes
Go to step 2. Refer to EN(H4SO
2.0) section. <Ref.
to EN(H4SO
2.0)(diag)-66, 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.
Inspect the rele- Replace the rear
vant DTC using oxygen sensor.
"List of Diagnostic <Ref. to FU(H4SO
Trouble Code 2.5)-35, Rear Oxy-
(DTC)". <Ref. to gen Sensor.>
EN(H4SO
2.5)(diag)-70, List
of Diagnostic Trou-
ble Code (DTC).>
NOTE:
In this case, it is
not necessary to
inspect DTC
P0139.
No