1991 LEXUS SC400 check engine

DIAGNOSTIC CHART
Check voltage of terminal VF.
Repair or replace harness or
connector.
Repair or replace.
Repair or replace.
Replace main oxygen sensor.
Repair or replace.
Repair or replace.
Replace main oxygen sensor.
Check voltage of terminal OX.
Check for open and short in harness and
connector.
Check each item found to be a possible
cause of problem.
Check compression.
Does malfunction disappear when a good
main oxygen sensor is installed?
Check and replace ECU.
Check each item found to be a possible
cause of problem.
Check compression.
Does malfunction disappear when a good
main oxygen sensor is instaled?
Check and replace ECU.
NG Type II
NG Type I
WIRING DIAGRAM
Refer to page TR±62 for the WIRING DIAGRAM.
TR±72
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ENGINE TROUBLESHOOTING Circuit Inspection
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NG
Type I
NG
Type II
NGOK
INSPECTION PROCEDURE
1Check voltage between terminals VF1, VF2 and E1 of check connector.
C
P
Cau-tion
1. Warm up engine at normal operating temperature
2. Connect terminals TE1 and E1 of check connector.
3. Connect positive prove to terminal VF1, VF2 and
negative prove to terminal E1 of check connector.
1. 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.
Go to step .
2Check voltage between 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 engine 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 .
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ENGINE TROUBLESHOOTING Circuit InspectionTR±73
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OKNG
OKNG
OKNG
NGYES
3Check for open and short in harness and connector between engine (& ECT) EC\
U and main
oxygen 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 r\
esults of the check in
or
 . The numbers in the table below show the order in which the checks shoul\
d be performed.
Main oxygen sensor
signal from either side
continues at 0 V.Main oxygen sensor
signals from both sides
continue at 0 V.
Possible CauseSee page
1Faulty sensor installation.±
3Injector circuitTR±128
23MisfireIG±6
4Valve timingEM±22
1Air leakageFI±7
2Fuel systemTR±110
6Characteristics deviation
in air flow meter.TR±82
4Characteristics deviation
in water temp. sensor.TR±64
5Characteristics deviation
in intake air temp. sensor.TR±68
Repair or replace.
5Check compression (See page EM±22).
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±74±
ENGINE TROUBLESHOOTING Circuit Inspection
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OKNG
OKNG
NGYES
7Check each item found to be a possible cause of problem.
Check each circuit found to be a possible cause of trouble according to the r\
esults of the check in
. The
numbers in the table below show the order in which the checks should be perform\
ed.
Main oxygen sensor
signal from either
side continues at 5 V.Main oxygen sensor signals from both
sides continue at 5 V.Main oxygen sensor signals from both
sides are normal.
Possible CauseSee page
17Injector circuitTR±128
3MisfireIG±6
24Valve timingEM±22
1Air leakageFI±7
12Fuel systemTR±110
48Characteristics deviation
in air flow meter.TR±82
25Characteristics deviation
in water temp. sensor.TR±64
36Characteristics deviation
in intake air temp. sensor.TR±68
Repair or replace.
8Check compression (See page EM±22).
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±75
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Diag. Code 24Intake Air Temp. Sensor Circuit
CIRCUIT DESCRIPTION
The intake air temp. sensor is built into the air flow meter and senses the\
intake air temperature.
The structure of the sensor and connection to the ECU is the same as in the\
water temp. sensor shown on page
TR±68.
If the ECU records the diagnostic code º24º, it operates the fail \
safe function, keeping the intake air temperature
at a constant 20 5C (68 5F).
Code No.Diagnostic Code Detecting ConditionTrouble Area
24Open or short in intake air temp. sensor circuit
for 0.5 sec. or more.
Open or short in intake air temp. sensor circuit.

Intake air temp. sensor

ECU
DIAGNOSTIC CHARTDIAGNOSTIC CHART
HINT: If diagnostic codes º22º (water temperature sensor circuit), º\
24º (intake air temperature sensor
circuit) and º41º (throttle position sensor circuit) are output simultaneousl\
y, E2 (sensor ground) may
be open.
WIRING DIAGRAM
Check and replace ECU. Replace water temp. sensor.
Check for momentary interruption
(See page
TR±23).
Repair or replace harness or
connector.
Check voltage of sensor.
Check resistance of sensor.
Check for open and short in harness and
connector between ECU and water temp. sensor.
±
ENGINE TROUBLESHOOTING Circuit InspectionTR±72
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NGOK
OKNG
OKNG
INSPECTION PROCEDURE
1Check voltage between terminals THA and E2 of engine & ECT ECU connector.
C
OK
P(2) Connect the Check Harness A.(See page TR±30)
(2) Turn ignition switch on.
Measure voltage between terminals THA and
E2 and engine & ECT ECU connector.
Check for momentary Intertuption
(See page TR±23).
2Check intake air temp. sensor.
C
OK
PDisconnect the air flow meter connector.
Measure resistance between terminals 1 and 2 of air
flow meter connector.
Resistance is within Acceptable Zone on chart.
Replace intake air temp. sensor (Replace air flow meter).
3
Check for open and short in harness and connector between engine & ECT ECU an\
d intake air
temp. sensor (See page IN±27).
Repair or replace harness or connector.
Check and replace engine & ECT ECU.
TR±73±
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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 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±
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. codeis no longer detected.
Malfunction: Open or Short in Injector circuit, Injector Leak or Blockage, Loose E/G Earth Bolt.

Initiate test mode (See page TR±12).
Start engine and warm up.
After engine is warmed up, let it idle for 5 min.
(After the engine is started, do not depress the accelerator pedal.)
\bIf the malfunction is not detected during idling, perform racing without a\
ny load at approx.
2,000 rpm for 30 sec.
HINT: If a malfunction exists, the ºCHECKº engine warning light will light up during the 5 min. idling
period or within 30 sec. of starting racing.
NOTICE: If the conditions in this test are not strictly followed, detection of the\
malfunction will not be possible.
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ENGINE TROUBLESHOOTING Circuit InspectionTR±76
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