Lexus SC300 / Lexus SC400. Manual - part 927

Diag. Code  27, 29

Sub–Oxygen Sensor Circuit

CIRCUIT DESCRIPTION

The sub–oxygen sensor is installed on the exhaust pipe. Its construction and operation is the same as the main
oxygen sensor on page 

TR–62

.

Code No.

Diagnostic Code Detecting Condition

Trouble Area

(1) Open or short in heater circuit of sub–oxygen

sensor for 0.5 sec. or more.

Open or short in heater circuit of sub–oxygen

sensor

Sub–oxygen sensor heater.

ECU

27

29

(2) Main oxygen sensor signal is 0.45 V or more and

sub–oxygen sensor signal is 0.45 V or less 

under sonditions (a) and (b).

(2 trip detection logic) *

(a) Coolant temp.: 80

°

C (176

°

F) or more.

(b) Accel. pedal: Fully depressed for 2 sec. or

more.

Open or short in sub–oxygen sensor circuit.

Sub–oxygen sensor

ECU

*: See 

page 

TR–21

.

HINT: 

Diag. code 27 is for the left bank sub–oxygen sensor circuit. 
Diag. code 29 is for the right bank sub–oxygen sensor circuit.

TR–82

–

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 Sub–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.



With the A/C ON and the transmission in D range (O/D OFF), drive at 40 –60 mph (64–96
km/h) for 6 min.



After the driving in , stop the vehicle.



Once the vehicle is stopped, within the next minute apply acceleration with the throttle
fully open for 4 sec.

HINT:  If a malfunction exists, the ”CHECK” engine warning light will light up after full acceleration

for 4 sec.

NOTICE: 

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

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–83

DIAGNOSTIC CHART

DIAGNOSTIC CHART

HINT:

•

When other codes are output in addition to 27 and 29 at the same time, check the circuits for other
codes first.

•

If diag. code 27 is displayed, check left bank sub–oxygen sensor circuit, and if 29 is displayed,
check right bank sub–oxygen sensor circuit.

WIRING DIAGRAM

Check voltage of terminals HTL2, HTR2.

Check resistance of sub–oxygen sensor heater.

Check and repair sub–oxygen sensor heater
circuit.

Check operation of sub–oxygen sensor heater.

Check and replace ECU.

Replace main oxygen sensor.

*: In this case, oxygen sensor can be

deteriorated.

Replace main oxygen sensor.*

TR–84

–

ENGINE TROUBLESHOOTING

Circuit Inspection

NG

OK

OK

NG

INSPECTION PROCEDURE

1

Disconnect the engine & ECT ECU connector.

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 between terminals HTL2, HTR2 of en-
gine & ECT ECU connector and body ground.

Voltage:

10 – 14 V

Go to step [3].

2

Check sub–oxygen sensor heater.

C

OK

P

Disconnect sub–oxygen sensor connector.

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

Resistance:

5.1 – 6.3 

 20



C (68



F)

Replace sub–oxygen sensor.

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

–

ENGINE TROUBLESHOOTING

Circuit Inspection

TR–85