Lexus SC300 / Lexus SC400. Manual - part 669

DIAGNOSTIC CHART

DIAGNOSTIC CHART

WIRING DIAGRAM

HINT:  If diag. code ”21” is output, check the left bank main oxygen sensor circuit. If diag. code ”28” 

is output, check the right bank main oxygen sensor circuit.

Check voltage of terminals HTL1, HTR1.

Check resistance of oxygen sensor heater.

Check and repair oxygen sensor heater circuit.

Check operation of oxygen sensor heater.

Check and replace ECU.

Replace main oxygen sensor.

*: In this case, oxygen sensor can be

deteriorated.

Replace main oxygen sensor.*

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–64

OK

NG

OK

NG

INSPECTION PROCEDURE

1

Disconnect the engine & ECT ECU connector, check voltage between terminals HTL1, HTR1 of
engine & ECT ECU connector and body ground.

C

OK

P

(2) Connect the Check Harness A.

(See page 

TR–30

)

(2) Disconnect engine & ECT ECU connector.

(2) Turn ignition switch on.

Measure voltage beween terminals HTL1, HTR1 of en-
gine & ECT ECU connector and body ground.

Voltage:

10 – 14 V

Go to step [3].

2

Check main oxygen sensor heater.

C

OK

P

Disconnect main oxygen sensor connector.

Measure resistance between terminals 1 and 2 of main
oxygen sensor connector.

Resistance:

5.1 – 6.3 

 at 20



C (68



F)

Replace main oxygen sensor.

Check and repair harness or connector between
main relay and main oxygen sensor, main oxygen
sensor and engine & ECT ECU.

TR–65

–

ENGINE TROUBLESHOOTING

Circuit Inspection

OK

NG

3

Check voltage between terminals HTL1, HTR1 of engine & ECT ECU connector and body
ground.

C

OK

Hint

P

(1) Connect engine & ECT ECU connector.

(2) Warm up engine to normal operating temperature.

Measure voltage between terminals of HTL1, HTR1 of
engine & ECT ECU connector and body ground, when
engine is idling and racing at 4,000 rpm.

In the 4,000 rpm racing check, continue engine racing
at 4,000rpm for approx. 20 seconds or more.

Replace main oxygen sensor.*

*: In this case, oxygen sensor can be deteriorated.

Check and replace engine & ECT ECU.

Reference  |  INSPECTION USING OSCILLOSCOPE

•

With the engine racing (4,000 rpm) measure between
terminals OXL1, OXR2 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–66

–MEMO–

TR–67

ENGINE TROUBLESHOOTING