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DI-42
DIAGNOSTICS - ENGINE (5VZ-FE)
DI5C4-08
DTC P0125 Insufficient Coolant 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 cata-
lytic 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 A/F sensor has the characteristic that provides output voltage* approximately proportional to the existing
air-fuel ratio. The A/F sensor output voltage* is used to provide feedback for the ECM to control the air-fuel
ratio. By the A/F sensor output, the ECM can determine the deviation amount from the stoichiometric air-fuel
ratio and control the proper injection time immediately. If the A/F sensor is malfunctioning, ECM is unable
to perform accurate air-fuel ratio control.
The A/F sensor is equipped with 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 low), current flows to the heater
to heat the sensor for accurate oxygen concentration detection.
*: The voltage value changes at the inside of the ECM only.
Ideal Air - Fuel Mixture
Atmosphere
Flange
Output Voltage
Platinum Electrode
Solid Electrolyte
Platinum Electrode
Heater
Coating (Ceramic)
Cover Richer - Air Fuel Ratio - Leaner
Exhaust Gas
P21242 FI7210 A00027
DTC No. DTC Detecting Condition Trouble Area
After engine is warmed up, A/F sensor output* does not S Open or short in A/F sensor (bank 1 sensor 1) circuit
change when conditions (a), (b) and (c) continue for at least S A/F sensor (bank 1 sensor 1)
1.5 min.: S Air induction system
*: Output value changes at inside of ECM only S Fuel pressure
P0125
(a) Engine speed: 1,500 rpm or more S Injector
(b) Vehicle speed: 40 - 100 km/h (25 - 62 mph) S Gas leakage on exhaust system
(c) Throttle valve does not fully closed S PCV valve and hose
(d) After starting engine y 140 sec. S ECM
HINT:
S After confirming DTC P0125, use the OBD II scan tool or TOYOTA hand-held tester to confirm voltage
output of the heated oxygen sensor (bank 1 sensor 1) from the CURRENT DATA.
S The ECM controls the voltage of the AF1+ and AF1- terminals of the ECM to the fixed voltage. There-
fore, it is impossible to confirm the A/F sensor output voltage without OBD II scan tool or TOYOTA
hand-held tester.
S OBD II scan tool (excluding TOYOTA hand-held tester) displays the one fifth of the A/F sensor output
voltage which is displayed on the TOYOTA hand-held tester.
2002 4RUNNER (RM887U)
Author : Date : 218
DI-43
DIAGNOSTICS - ENGINE (5VZ-FE)
WIRING DIAGRAM
ECM
B+
14 4
IG1 GR - B MREL
E14
GR - B 1 W-L D D W - L 13 W-L 14
W-L W AF1+
IG1 J7 J7 II4 E9
2 3
2 2 A27
J/C
1 3 A/F Sensor
21
EFI Relay
Engine L HTAF
1 4 E9
Room E04
R/B
2 5 26
W-L B AF1-
E9
2 2
2
W-B L-R
27
B OX2B
2 Engine E9
2
EFI
A J2 Room 3
1 R/B H4
J/C Heated Oxygen
Sensor (Bank 1 Sensor 2)
2
A J1 29 HT2B
W 1 4 G -Y
E9
W-B BR E03
EB
Battery EC
A15462
INSPECTION PROCEDURE
HINT:
Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool. Because freeze frame records
the engine conditions when the malfunction is detected. When troubleshooting, it is useful for determining
whether the vehicle was running or stopped, the engine was warmed up or not, the air-fuel ratio was lean
or rich, etc. at the time of the malfunction.
1 Are there any other codes (besides DTC P0125) being output?
YES Go to relevant DTC chart.
NO
2002 4RUNNER (RM887U)
Author : Date : 219
DI-44
DIAGNOSTICS - ENGINE (5VZ-FE)
2 Check connection of PCV valve and hose
NG Repair or replace PCV valve and hose
OK
3 Connect OBD II scan tool or TOYOTA hand-held tester, and read value for volt-
age output of A/F sensor (bank 1 sensor 1).
PREPARATION:
(a) Connect the OBD II scan tool or TOYOTA hand-held tester to the DLC3.
(b) Warm up the A/F sensor with the engine speed at 2,500 rpm for approx. 90 sec.
CHECK:
Read the voltage value of the A/F sensor on the screen of the OBD II scan tool or TOYOTA hand-held tester
when you perform all the following conditions.
HINT:
The voltage of AF1+ terminal of the ECM is 3.3 fixed and the AF1- terminal is 3.0 V fixed. Therefore, it is
impossible to check the A/F sensor output voltage at the terminals (AF1+/AF1-) of the ECM.
OK:
Condition A/F Sensor Voltage value
Engine idling
S N t remains at 3.3. V (0.660 V*)
Not i t 3 3 (0 660 V )
E i idling
Engine idli
S Not remains at 3.8 V (0.76 V*) or more
3 8 (0 76
Driving at engine speed 1,500 rpm or more and vehicle S Not remains at 2.8 V (0.56 V*) or less
speed 40 km/h (25 mph) or move, and operate throttle valve
*: When you use the OBD II scan tool (excluding TOYOTA hand-held tester)
open and close
HINT:
S During fuel enrichment, there is a case that the output voltage of the A/F sensor is below 2.8 V (0.56
V*), it is normal.
S During fuel cut, there is a case that the output voltage of the A/F sensor is above 3.8 V (0.76 V*), it is
normal.
S If the output voltage of the A/F sensor remains at 3.30 V (0.660 V*) even after performing all the above
conditions, the A/F sensor circuit may be open.
S If the output voltage of the A/F sensor remains at 3.8 V (0.76 V*) or more, or 2.8 V (0.56 V*) or less
even after performing all the above conditions, the A/F sensor circuit may be short.
*: When you use the OBD II scan tool (excluding TOYOTA hand-held tester).
OK Go to step 9.
NG
2002 4RUNNER (RM887U)
Author : Date : 220
DI-45
DIAGNOSTICS - ENGINE (5VZ-FE)
4 Check for open and short in harness and connector between ECM and A/F sen-
sor (bank 1 sensor 1) (See page IN-28 ).
NG Repair or replace harness or connector.
OK
5 Check resistance of A/F sensor heater (bank 1 sensor 1) (See page SF-51 ).
NG Replace A/F sensor.
OK
6 Check air induction system (See page SF-1 ).
NG Repair or replace.
OK
7 Check fuel pressure (See page SF-7 ).
NG Check and repair fuel pump, pressure regulator,
fuel pipe line and filter (See page SF-1 ).
OK
8 Check injector injection (See page SF-21 ).
NG Replace injector.
OK
2002 4RUNNER (RM887U)
Author : Date : 221
DI-46
DIAGNOSTICS - ENGINE (5VZ-FE)
9 Check gas leakage on exhaust system.
NG Repair or replace.
OK
Replace A/F sensor (bank 1 sensor 1).
10 Perform confirmation driving pattern (See page DI-122 ).
Go
11 Is there DTC P0125 being output again?
YES Check and replace ECM (See page IN-28 ).
NO
12 Did vehicle runs out of fuel in past?
NO Check for intermittent problems (See page
DI-3 ).
YES
DTC P0125 is caused by running out of fuel.
2002 4RUNNER (RM887U)
Author : Date : 222
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