1991 LEXUS SC400 oxygen sensor

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 c\
onfirming that diag. code is no longer detected.
Malfunction: Open or Short in Main Oxygen Sensor

Initiate test mode (See page TR±12).
Start engine and warm up.
After engine is warmed up, let it idle for 3 min.
Perform quick racing to 4,000 rpm three times by accelerator pedal.
After performing the racing in (4), perform racing at 2,000 rpm for 90\
sec.
HINT: If a malfunction exists, the ºCHECKº engine warning light will light up after 90 sec. from the start of racing.
NOTICE: If the conditions in this test are not strictly followed, detection of t\
he malfunction will not be possible.
TR±75±
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DIAGNOSTIC CHART
DIAGNOSTIC
CHART
WIRING DIAGRAM
Refer to page TR±64 for the WIRING DIAGRAM.
Check and replace ECU. Repair or replace harness or
connector.
Check compression Check and replace ECU.
Does malfunction disappear when a good main
oxygen sensor is installed?
Check compression.
Check each item found to be a possible cause
of problem. Check for open and short in harness and
connector.
Check voltage of terminal OX.
Check voltage of terminal VF.
Replace main oxygen sensor. Repair or replace.
Does malfunction disappear when a good main
oxygen sensor is installed? Check each item found to be a possible cause
of problem. Replace main oxygen sensor.
Repair or replace.
Repair or replace.
Repair or replace.
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NG
Type ING
Type II
NG
OK
INSPECTION PROCEDURE
1Check voltage between terminals VF1, VF2 and E1 of check connector.
C
P(2) Warm up engine at normal operating temperature.
(2) Connect terminals TE1 and E1 of check connector.
(2) Connect positive prove to terminal VF1, VF2 and negative prove to terminal E1 of check connector.
(2) Warm up the oxygen sensor by running engine at 2,500 rpm for about 2 minutes.
(2) Then, maintaining engine at 2,500 rpm, count how many times needle of voltmeter fluctuates between
0 and 5 V
Result:
Go to step [7].
2Check voltage beween terminals OX1, OX2 and E1 of check connector.
C
OK
Hint
PWarm up engine at normal operating temperature.
Measure voltage between terminals OX1, OX2 and E1
of check connector when sngine is suddenly raced to full
throttle.
The Voltage should be 0.5 V or higher at least once.
Perform inspection within 1 second
Go to step [7].
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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±
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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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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
±
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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±
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