Subaru 2009 IMPREZA Service Manual page 781

Diagnostic Trouble Code (DTC) Detecting Criteria
AH: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 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 from 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 from 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 from 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(H4SO)-57
GENERAL DESCRIPTION
(E)
(A)
(C) (B)
(C) Rich
(F) Comparative voltage
(3) Electromotive force
(F)
(D)
EN-01696