Subaru 2009 IMPREZA Service Manual page 1646

Diagnostic Trouble Code (DTC) Detecting Criteria
GENERAL DESCRIPTION
AF:DTC P0139 O2 SENSOR CIRCUIT SLOW RESPONSE (BANK 1 SENSOR 2)
1. OUTLINE OF DIAGNOSIS
Detect the slow response of the oxygen sensor.
Judge as NG if either of the rich to lean response diagnosis or lean to rich response diagnosis is NG, and
Judge as OK if both are OK.
[Rich lean diagnosis response]
1. Measure the response time for oxygen sensor output changes when the A/F ratio changes to rich to lean.
If the measured response time is larger than the threshold value, it is NG. If it is smaller, it is OK.
2. Judge as NG when the oxygen sensor voltage is large (rich) when recovering from a deceleration fuel cut.
[Lean rich diagnosis response]
1. Measure the response time for oxygen sensor output changes when the A/F ratio changes to lean to rich.
If the measured response time is larger than the threshold value, it is NG.
2. Judge as NG when the oxygen sensor voltage remains small when recovering from a deceleration fuel
cut.
DIAGNOSTIC METHOD
Measure the response time of the output change of the oxygen sensor when the A/F ratio changes to rich to
lean. And Judge as NG when the measured response time is larger than the threshold value.
2. COMPONENT DESCRIPTION
(3)
V
(2)
(A) Electromotive force
(D) Lean
(1) Atmosphere
(1)
(2)
(B) Air fuel ratio
(E) Theoretical air fuel ratio
(2) Exhaust gas
GD(H4DOTC)-58
(E)
(A)
(C) (B)
(C) Rich
(F) Comparative voltage
(3) Electromotive force
(F)
(D)
EN-01696