Lexus SC300 / Lexus SC400. Manual - part 678

Diag. Code   43

Starter Signal Circuit

CIRCUIT DESCRIPTION

When the engine is cranked, the intake air flow is slow, so fuel vaporization is poor. A rich mixture is therefore
necessary in order to achieve good startability. While the engine is being cranked, the battery voltage is applied
to terminal STA of the ECU. The starter signal is mainly used to increase the fuel injection volume for the starting
injection control and after–start injection control.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

43

No starter signal to ECU

Open or short in starter signal circuit.

Open or short in ignition switch or main relay

circuit.

ECU

DIAGNOSTIC CHART

DIAGNOSTIC CHART

HINT: 

This diagnostic chart is based on the premise that the engine is cranked normally. If the engine is not
cranked, proceed to the matrix chart of problem symptoms on page 

TR–35

.

Check for the test mode.

Check for open in harness and connectors
between ECU and relay.

Check and replace ECU.

Proceed to next circuit inspection shown
on matrix chart (See page 

TR–35

).

Repair or replace harness or
connector.

WIRING DIAGRAM

TR–100

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ENGINE TROUBLESHOOTING

Circuit Inspection

NG

OK

OK

NG

v

INSPECTION PROCEDURE

1

Check output condition of diag. code 43.

C

OK

P

OK

C

Setting the test mode.

(2) Turn ignition switch OFF.

(2) Connect terminals TE2 and E1 of TDCL.

(2) Turn ignition switch ON.

(Don’t start the engine)

(2) Connect terminals TE1 and E1 of TDCL.

Check if code “43” is output by the “CHECK” engine
warning light.

Code “43” is output.

Start the engine.
Check if the code “43” disapears.

Code “43” is not output.

Proceed to next circuit inspection shown on maxtric
chart (See page 

TR–35

).

2

Check for open in harness and connectors between engine & ECT ECU and starter relay

(See page 

IN–27

).

Repair or replace harness or connector.

Check and replace engine & ECT ECU.

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ENGINE TROUBLESHOOTING

Circuit Inspection

TR–101

Diag. Code  52,53,55 Knock Sensor Circuit

CIRCUIT DESCRIPTION

Knock sensors are fitted one each to the right bank and left bank of the cylinder block to detect engine knocking.
This sensor contains a piezoelectric element which generates a voltage when it becomes deformed, which oc-
curs when the cylinder block vibrates due to knocking. If engine knocking occurs, ignition timing is retarded to
suppress it.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

52

No No. 1 knock sensor signal to ECU for 3 crank revolu-

tions with engine speed between 1,600 rpm  5,200 rpm.

Open or short in No. 1 knock sensor circuit.

No. 1 knock sensor (looseness)

ECU

53

Engine control computer (for knock control) malfunc-

tion at engine speed between 650 rpm and 5,200 rpm.

ECU

55

No No. 2 knock sensor signal to ECU for 3 crank revolu-

tions with engine speed between 1,600 rpm  5,200 rpm.

Open or short in No. 2 knock sensor circuit.

No. 2 knock sensor (looseness)

ECU

If the ECU detects the above diagnosis conditions, it operates the fail safe function in which the corrective retard
angle value is set to the maximum value.

TR–102

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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.

Malfunction: Open or Short in Knock Sensor

Start engine and warm up.



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



With the A/C ON, perform quick racing (5,000 rpm) three times.
(Rapidly depress the accelerator pedal and suddenly release it.)

HINT: 

If a malfunction exists, the ”CHECK” engine warning light will light up when sudden racing is 
performed.

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 Inspection

TR–103