Toyota ZZT230 series, Repair Manual

Page: 255 / 821

01-44
-
DTC P0125 Insufficient Temp. for Closed Loop
Fuel Control
CIRCUIT DESCRIPTION
To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three-way
catalytic converter is used, but for the most efficient use of the three-way catalytic converter, the air-fuel
ratio must
be
precisely controlled so that it is always close to the stoichiometric air-fuel ratio.
The oxygen sensor has the characteristic whereby its output voltage changes suddenly in the vicinity of the
stoichiometric air-fuel ratio. This is used to detect the oxygen concentration in the exhaust gas and provide
feedback to the computer for control of the air-fuel ratio.
When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust increases and the oxygen
sensor Informs the ECM of the LEAN condition (small electromotive force: < 0.45 V).
When the air-fuel ratio is RICHER than the stoichiometric air-fuel ratio the oxygen concentration in the ex­
haust gas in reduced and the oxygen sensor informs the ECM of the RICH condition (large electromotive
force:
>
0.45V).
The ECM judges by the electromotive force from the oxygen sensor whether the air-fuel ratio is RICH or
LEAN and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes out­
put of abnormal electromotive force, the ECM is unable to perform accurate air-fuel ratio control.
The oxygen sensors include a heater which heats the zirconia element The heater is controlled by the ECM.
When the intake air volume is low (the temp. of the exhaust gas is tow) current flows to the heater to heat
the sensor for accurate oxygen concentration detection.
DTC No.
P0125
Atmosphere
Ideal Air-Fuel Mixture
Platinum Electrode
j
Exhaust Gas
DTC Detecting Condition
After
the engine is warmed up, oxygen sensor (bank 1 sensor
1)
output
does
not
Indicate
RICH
( � 0.45 V) even once when
conOtions (a), (b),
and
(c)
continue
lor
at leasl 1.5 min.
(e)
Engine
speed;
1,500
rpm
or more
{b) Vehicle
speed;
40 -
1 00
krrutl
(25- 62 mph)
{c) Throttle valve
does
not
fully closed
Richer-Air Fuel Ratio-Leaner
• Fuel system
• Air induction system
• Injector
• Ignition system
Trouble Area
• Gas leakage on exhaust system
•Open
or
short in heated oxygen sensor (bank 1 sensor 1 )
circu�
• Heated oxygen sensor (bank 1 sensor 1)
•ECM