Subaru IMPREZA 2004 Service Manual page 64

Diagnostic Procedure with Diagnostic Trouble Code (DTC)
Step
1 CHECK ANY OTHER DTC ON DISPLAY.
AB:DTC P0171 SYSTEM TOO LEAN (BANK 1)
Refer to DTC P0172 for diagnostic procedure. <Ref. to EN(H4SO)(diag)-142, DTC P0172 SYSTEM TOO
RICH (BANK 1), Diagnostic Procedure with Diagnostic Trouble Code (DTC).>
Check
Is any other DTC displayed?
EN(H4SO)(diag)-141
ENGINE (DIAGNOSTICS)
Yes
Check DTC using Replace the rear
"List of Diagnostic oxygen sensor.
Trouble Code <Ref. to
(DTC)". <Ref. to FU(H4SO)-43,
EN(H4SO)(diag)- Front Oxygen (A/
74, List of Diag- F) Sensor.>
nostic Trouble
Code (DTC).>
NOTE:
In this case, it is
not necessary to
inspect
DTC P0139.
No