1991 LEXUS SC400 sensor

OKNG
OKNG
OKNG
NOYES
3
Check for open and short in harness and connector between engine & ECT ECU an\
d main oxy-
gen sensor, engine & ECT ECU and check connector (See page IN±27).
Repair or replace harness or connector.
4Check each item found to be a possible cause of problem.
Check each circuit found to be a possible cause of trouble according to the results of the check in \
1 or
2 . The numbers in the table below show the order in which the checks sho\
uld be preformed.
TR±132
TR±6
TR±51
TR±13
TR±114
TR±88
TR±68
TR±72
Repair or replace.
5Check comparession (See page EM±30).
Repair or replace.
6Does malfunction disappear when a good main oxygen sensor is installed?
Replace main oxygen sensor.
check and replace engine & ECT ECU.
TR±79±
ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
OKNG
NOYES
7Check each item found to be a possible cause of problem.
Check each ciruit found to be a possible cause of trouble according to the re\
sults of the check in 1. The
numbers in the table below show the order in the which the checks should be per\
formed.
TR±132
TR±6
TR±51
TR±13
TR±114
TR±88
TR±68
TR±72
TR±72
Repair or replace.
8Check compression (See page EM±30).
Repair or replace.
9Does malfunction disappear when a good main oxygen sensor is installed?
Replace main oxygen sensor.
Check and replace engine & ECT ECU.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±80
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Diag. Code 27Sub±Oxygen Sensor Circuit (Only for
USA spec.)
CIRCUIT DESCRIPTION
The sub±oxygen sensor is installed on the exhaust pipe. Its construction\
and operation is the same
as the main oxygen sensor on page TR±60.
Code No.
Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in heater circuit of sub± oxygen sensor for 0.5 sec. or more.
Open or short in heater circuit of sub±oxy-
gen sensor.
 Sub±oxygen sensor heater.
 ECU
27
(2) Main oxygen sensor signal is 0.45 V or
more and sub±oxygen sensor signal is
0.45 V or less under conditions (a) (c).
(2 trip detection logic) *
(a) Coolant temp.: 80 °C (176 °F)
or more.
(b) Engine speed: 1,500 rpm or more.
(c) Accel. pedal: Fully depressed for 2 sec. or more.
Open or short in sub±oxygen sensor cir-
cuit.
 Sub±oxygen sensor
 ECU
* : See page TR±25.
TR±76
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ENGINE TROUBLESHOOTING Circuit Inspection
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CIRCUIT DESCRIPTION (Cont'd)
DIAGNOSIS CODE DETECTION DRIVING PATTERN
Purpose of the driving pattern.
(a) To simulate diag. code detecting condition after diag. code is recorded.
(b) To check that the malfunction is corrected when the repair is completed co\
nfirming that diag. code isno longer detected.

Disconnect the EFI fuse (15 A) for 10 sec. or more, with IG switch OFF\
.
Initiate test mode (Connect terminal TE2 and E1 of check connector with\
IG switch
OFF).
Start the engine and warm up with all ACC switch OFF.
After the engine is warmed up, let it drive at 50 55 mph (80 88 km/h) for\
10 min. or
more.
After driving, stop at a safe place and perform idling for 2 min. or les\
s.
After performing the idling in , perform acceleration to 60 mph (96 km/h) with the
throttle valve fully open.
HINT: If a malfunction exists, the ºCHECKº engine warning light will lig\
ht up during step
.
NOTICE: If the conditions in this test are not strictly followed, detection of t\
he malfunction will not be possible.
Malfunction: Open or Short in Sub±Oxygen Sensor
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±77
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DIAGNOSTIC CHART
HINT: When other codes are output in addition to 27 at the same time, check the ci\
rcuits for othercodes first.
WIRING DIAGRAM
Check voltage between terminal HT.
Check resistance of sub±oxygen sensor
heater.
Check for open and short in harness and
connector between EFI main relay and ECU.
Check and replace ECU.
Check operation of sub±oxygen sensor
heater.
Check and replace ECU.
Replace sub±oxygen sensor.
Repair or replace harness or
connector.
Replace sub±oxygen sensor.*
*: In this case, oxygen sensor can bedeteriorated.
TR±78±
ENGINE TROUBLESHOOTING Circuit Inspection
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NGOK
OKNG
INSPECTION PROCEDURE
1Check voltage between terminal HT of engine (&ECT) ECU connector and body g\
round.
C
OK
P1. Connect the Check Harness A. (See page TR±34).
2. Turn ignition switch on.
Measure voltage between terminal HT of engine (&
ECT) ECU connector and body ground.
Voltage: 10 ± 14 V
Go to step
.
2Check sub±oxygen sensor heater.
C
OK
PDisconnect sub±oxygen sensor connector.
(See page FI±78).
Measure resistance between terminals 1 and 2 of
sub±oxygen sensor connector.
Resistance: 5.1 ± 6.3
20
°C (68°F)
Replace sub±oxygen sensor.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±79
WhereEverybodyKnowsYourName

OKNG
NGOK
3Check for open and short in harness and connector between EFI main relay and engine (& ECT)
ECU (See page IN±27).
Repair or replace harness or connector.
Check and replace engine (& ECT) ECU.
4Check voltage between terminal HT of engine (& ECT) ECU connector and \
body ground.
C
OK
PWarm up engine to normal operating temperature.
Measure voltage between terminal HT of engine (&
ECT) ECU connector and body ground, when engine is
idling and racing at 3,500 rpm.
Replace sub±oxygen sensor.* *: In this case, oxygen sensor can be deteriorated.
Check and replace engine (& ECT) ECU.
TR±80±
ENGINE TROUBLESHOOTING Circuit Inspection
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Diag. Code 27, 29Sub±Oxygen Sensor Circuit
CIRCUIT DESCRIPTION
The sub±oxygen sensor is installed on the exhaust pipe. Its construction\
and operation is the same as the main
oxygen sensor on page TR±62.
Code No.Diagnostic Code Detecting ConditionTrouble Area
(1) Open or short in heater circuit of sub±oxygen
sensor for 0.5 sec. or more.

Open or short in heater circuit of sub±oxygen
sensor

Sub±oxygen sensor heater.

ECU
27


29(2) Main oxygen sensor signal is 0.45 V or more and sub±oxygen sensor signal is 0.45 V or less
under sonditions (a) and (b).
(2 trip detection logic) *
(a) Coolant temp.: 80 5C (176 5F) or more.
(b) Accel. pedal: Fully depressed for 2 sec. or more.

Open or short in sub±oxygen sensor circuit.

Sub±oxygen sensor

ECU
*: See page TR±21.
HINT: Diag. code 27 is for the left bank sub±oxygen sensor circuit.
Diag. code 29 is for the right bank sub±oxygen sensor circuit.
TR±82
±
ENGINE TROUBLESHOOTING Circuit Inspection
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