Lexus SC300 / Lexus SC400. Manual - part 918

Reference 

INSPECTION USING OSCILLOSCOPE

•

With the engine racing (4,000 rpm) measure
between terminals OX1, OX2 and E1 of engine (&
ECT) ECU.

HINT: The correct waveform appears as shown in the

illustration on the left, oscillating between approx.
0.1 V and 0.9 V

If the oxygen sensor is deteriorated, the amplitude
of the voltage is reduced as shown on the left.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–63

Diag. Code   16

ECT Control Signal Malfunction

CIRCUIT DESCRIPTION

The signal from the ECT CPU retards the ignition timeing of the engine during ECT gear shifting, thus mo-
mentarily reducing torque output of the engine for smooth clutch operation inside the transmission and re-
duced shift shock.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

16

Fault in communications between the engine

CPU and ECT CPU in the ECU.

ECU

If the ECU records the diagnostic code “16” in memory, it prohibits the torque control of the ECT which per-
forms smooth gear shifting.

DIAGNOSTIC CHART

DIAGNOSTIC CHART

Are there any other codes (besides Code 16)
being output?

Replace engine & ECT ECU.

Go to relevant diag. code chart.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–61

Diag. Code  21, 28

Main Oxygen Sensor Circuit

CIRCUIT DESCRIPTION

To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three–way
catalyst is used, but for most efficient use of the three–way catalyst, 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 characteristic is used to detect the oxygen concentration in the ex-
haust 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 ECU of the LEAN condition (small electromotive force: O V).
When the air–fuel ratio is RICHER than the stoichiometric air–fuel ratio the oxygen concentration in the
exhaust gas is reduced and the oxygen sensor informs the ECU of the RICH condition (large electromotive
force: 1V).
The ECU 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
output of abnormal electromotive force, the ECU is unable to perform accurate air–fuel ratio control.
The main oxygen sensors include a heater which heats the Zirconia element. The heater is controlled by
the ECU. When the intake air volume is low (the temperature of the exhaust gas is low) current flows to
the heater to heat the sensor for accurate oxygen concentration detection.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

(1) Open or short in heater circuit of main
oxygen sensor for 0.5 sec. or more.

Open or short in heater circuit of main oxygen sen-

sor.

Main oxygen sensor heater

ECU

21

⋅

28

(2)

Main oxygen sensor signal voltage is reduced to be-
tween 0.35 V and 0.70 V for 60 sec. under conditions
(a) ~ (d). (2 trip detection logic)*

(a)  Coolant temp.: Between 70

°

C (158

°

F) and 95

°

C

(203

°

F).

(b)  Engine speed: 1,500 rpm or more.
(c)  Load driving (EX. ECT in 4th speed, A/C ON, Flat

road, 50 mph (80km/h)).

(d)  Main oxygen sensor signal voltage: 

Alternating above and below 0.45 V.

Main oxygen sensor circuit

Main oxygen sensor

*: See page 

TR–21

.

HINT: Diag. code 21 is for the left bank main oxygen sensor circuit. Diag. code 28 is for the right bank main oxy-

gen sensor circuit.

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–62

CIRCUIT DESCRIPTION (Cont’d)

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 confirming that diag.

code is no longer detected.

DIAGNOSIS CODE DETECTION DRIVING PATTERN



 

Initiate test mode (See page 

TR–12

).



 

Start engine and warm up.



 

After engine is warmed up, let it idle for 3 min.



With the A/C ON and the transmission in D range (O/D ON), perform
gradual acceleration with in the range 1,300 – 1,700 rpm (centered
around 1,500 rpm).
(Take care that the engine speed does not fall below 1,200 rpm when
shifting. Gradually depress the accelerator pedal and kept it. Steady so
that engine braking does not occur.)



 

Maintain the vehicle speed at 40 – 50 mph (64 – 80 km/h).



 

Keep the vehicle running for 1 – 2 min. after starting acceleration.

HINT:

If a malfunction exists, the ”CHECK” engine warning light will light up af-
ter approx. 60 sec. from the start of acceleration.

NOTICE: 

If the conditions in this test are not strictly followed, detection
of the malfunction will not be possible.

TR–63

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ENGINE TROUBLESHOOTING

Circuit Inspection