Lexus SC300 / Lexus SC400. Manual - part 917

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

Replace engine (& ECT) ECU.

Go to relevant diag. code
chart.

Diag. Code 16

ECT Control Signal Malfunction

CIRCUIT DESCRIPTION

The signal from the ECT CPU retards the ignition timing of the engine during ECT gear shifting, thus momentarily
reducing torque output of the engine for smooth clutch operation inside the transmission and reduced shift
shock.
If the ECU records the diagnostic code ”16” in memory, it prohibits the torque control of the ECT which performs
smooth gear shifting.

DTC No.

DTC 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 trouble code ”16” in memory, it prohibits the torque control of the ECT which
performs smooth gear shifting.

DIAGNOSTIC CHART

DIAGNOSTIC CHART

TR–58

–

ENGINE TROUBLESHOOTING

Circuit Inspection

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 stoi-
chiometric air–fuel ratio. This characteristic is used to detect the oxygen concentration in the exhaust 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 sen-
sor informs the ECU of the LEAN condition (small electromotive force: 0 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 ab-
normal electromotive force, the ECU is unable to perform accurate air–fuel ratio control.

DTC No.

DTC Detecting Condition

Trouble Area

21

.

28

(1) Main oxygen sensor signal voltage is reduced

to tbetween 0.35 V and 0.70 V for 60 sec. under
conditions (a) 

 (d). (2 trip detection logic)*

(a) Coolant temp.:  Between 80

°

C (176

°

F) and

95

°

C (203

°

F).

(b) Engine speed: 1,500 rpm or more

(c) Load driving (EX. ECT in 4th (5th for M/T)

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

(2) Main oxygen sensor signal voltage exceeds

0.70 V for 3 sec. or more during fuel cut.

*: See page 

TR–25

..

HINT: Diag. trouble code 21 is for the front side main oxygen sensor circuit. Diag. trouble code 28 is for the rear

side main oxygen sensor circuit.

TR–60

–

ENGINE TROUBLESHOOTING

Circuit Inspection

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 confirming that diag. code is

no 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 or TDCL with IG
switch OFF).



Start the engine and warm up with all ACC switch OFF.



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



After performing the idling in , perform gradual acceleration with in the range 1,300
1,700 rpm (centered around 1,500 rpm) with the A/C switch ON and D range for A/T
(5th for M/T).
(Take care that the engine speed does not fall below 1,200 rpm when shifting. Gradu-
ally 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 after 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.

Malfunction: Main Oxygen Sensor Deterioration

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–61

DIAGNOSTIC CHART

Are there any other codes (Besides 21 or
28) being output?

Replace oxygen sensor.

Go to relevant diag. code chart.

HINT: If diag. code 21 is output, replace the front side main oxygen sensor.

If diag. code 28 is output, replace the rear side main oxygen sensor.

WIRING DIAGRAM

TR–62

–

ENGINE TROUBLESHOOTING

Circuit Inspection