Subaru 2004 LEGACY Service Manual page 94

Diagnostic Procedure with Diagnostic Trouble Code (DTC)
Step
5 CHECK EXHAUST SYSTEM.
Check exhaust system parts.
NOTE:
Check the following items:
• Loose part of exhaust system and incom-
plete installation
• Damage (crack, hole etc.) of parts
• Looseness and ill fitting of parts between
front oxygen (A/F) sensor and rear oxygen
sensor
AO:DTC P0171 SYSTEM TOO LEAN (BANK 1)
Refer to DTC P0175 for diagnostic procedure. <Ref. to EN(H6DO)(diag)-164, DTC P0175 SYSTEM TOO
RICH (BANK 2), Diagnostic Procedure with Diagnostic Trouble Code (DTC).>
AP:DTC P0172 SYSTEM TOO RICH (BANK 1)
Refer to DTC P0175 for diagnostic procedure. <Ref. to EN(H6DO)(diag)-164, DTC P0175 SYSTEM TOO
RICH (BANK 2), Diagnostic Procedure with Diagnostic Trouble Code (DTC).>
AQ:DTC P0174 SYSTEM TOO LEAN (BANK 2)
Refer to DTC P0175 for diagnostic procedure. <Ref. to EN(H6DO)(diag)-164, DTC P0175 SYSTEM TOO
RICH (BANK 2), Diagnostic Procedure with Diagnostic Trouble Code (DTC).>
Check
Is there any fault in exhaust
system?
EN(H6DO)(diag)-163
ENGINE (DIAGNOSTICS)
Yes
Repair or replace Replace the rear
the faulty part. oxygen sensor.
<Ref. to
FU(H6DO)-32,
Rear Oxygen Sen-
sor.>
No