Lexus RX300 (MCU15). Service manual - part 202

A02021

ON

VTA1 (+) E2 (–)

A02022

ON

E2 (–)

VC (+)

DI–40

–

DIAGNOSTICS

ENGINE

LEXUS RX300   (RM785E)

When not using hand–held tester:

1

Check voltage between terminals VTA1 and E2 of engine ECU

PREPARATION:
(a)

Remove the engine ECU with connector still connected
(See page FI–70).

(b)

Turn the ignition switch ON.

CHECK:
Measure voltage between terminals VTA1 and E2 of the engine
ECU connector.
OK:

Throttle valve

Voltage

Fully closed

0.3 – 1.0 V

Fully open

2.7 – 5.2 V

NG

Check and replace engine ECU (See page
IN–32).

OK

Check for open and short in harness and connector between engine ECU and throttle position
sensor (VTA line) (See page IN–32).

2

Check voltage between terminals VC and E2 of engine ECU connector.

PREPARATION:
(a)

Remove the engine ECU with connector still connected
(See page FI–70).

(b)

Turn the ignition switch ON.

CHECK:
Measure voltage between terminals VC and E2 of the engine
ECU connector.
OK:

Voltage: 4.5 – 5.5 V

NG

Check and replace engine ECU (See page
IN–32).

OK

Check for open in harness and connector be-
tween engine ECU and sensor (VC line) (See
page IN–32).

–

DIAGNOSTICS

ENGINE

DI–41

LEXUS RX300   (RM785E)

DTC

P0121

Throttle/Pedal Position Sensor/Switch ”A”
Circuit Range/Performance Problem

CIRCUIT DESCRIPTION

Refer to DTC P0120 onpage DI-36.

DTC No.

DTC Detecting Condition

Trouble Area

P0121

While vehicle speed drops from 30 km/h (19 mph) or more to 0

km/h (0 mph), output value of throttle position sensor is out of

applicable range. (2 trip detection logic)

S Throttle position sensor

S Engine ECU

INSPECTION PROCEDURE

HINT:
Read freeze frame data using hand–held tester. Because freeze frame records the engine conditions when
the malfunction is detected, when troubleshooting it is useful for determining whether the vehicle was run-
ning or stopped, the engine warmed up or not, the air–fuel ratio lean or rich, etc. at the time of the malfunction.

Replace throttle body (See page FI–33).

DI1LI–10

A00477

Cover

Atmosphere

Platinum

Electrode

Solid Electrolyte

(Zirconia Element)

Platinum

Electrode

Heater

Coating (Ceramic)

Exhaust Gas

(V)
4.0

3.8

3.6

3.4
3.2

3.0

2.8
2.6

17

Air–Fuel Ratio

2.4

16

15

12

14

13

19

18

engine

 ECU

Monitored

A/F

 Sensor V

oltage

DI–42

–

DIAGNOSTICS

ENGINE

LEXUS RX300   (RM785E)

DTC

P0125

Insufficient  Coolant Temp. for Closed Loop
Fuel Control

CIRCUIT DESCRIPTION

To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three–way
catalytic converter is used. For the most efficient use of the three–way catalytic converter, the air–fuel ratio
must be precisely controlled so that it is always close to the stoichiometric air–fuel ratio.
The A/F sensor has the characteristic that provides output voltage

*

 approximately proportional to the existing

air–fuel ratio. The A/F sensor output voltage

*

 is used to provide feedback for the engine ECU to control the

air–fuel ratio.
By the A/F sensor output, the engine ECU can determine the deviation amount from the stoichiometric air–
fuel ratio and control the proper injection time immediately. If the A/F sensor is malfunctioning, the engine
ECU is unable to perform accurate air–fuel ratio control.
The A/F sensor is equipped with a heater which heats the zirconia element. The heater is controlled by the
engine ECU. When the intake air volume is low (the temp. of the exhaust gas is low), current flows to the
heater to heat the sensor for accurate oxygen concentration detection.
*: The voltage value changes at the inside of the engine ECU only.

DTC No.

DTC Detecting Condition

Trouble Area

P0125

After engine is warmed up, A/F sensor output* does not

change when conditions (a), (b) and (c) continue for at least

1.5 min.

*: Output value changes at inside of engine ECU only.

(a) Engine speed:   1,500 rpm or more

(b) Vehicle speed:  40 ∼ 100 km/h (25 ∼ 62 mph)

(c) Throttle valve does not fully closed

(d) After starting engine y 140 sec

S Fuel system

S Injector

S Ignition system

S Gas leakage on exhaust system

S Open or short in A/F sensor circuit (bank 1, 2 sensor 1)

S A/F sensors (bank 1, 2 sensor 1)

S Engine ECU

DI7J3–03

A14513

Engine ECU

MREL

E9

W–B

2

3

4

B–W

8

W

19

B

2

4

EC

A

B–R

Battery

EFI Relay

1

2

3

5

1A

2G

8

2I

A/F HEATER Relay

Engine Room J/B

H12
Oxygen Sensor 
(Bank 1 Sensor 2)

A

5

2B

2G

2F

2

2

2

2

5

EFI

B–R

A

B–R

J10
J/C

2

2

2

C

3

1

1

2

4

ED

(Bank 2 Sensor 1)

A12 A/F Sensor

(Bank 1 Sensor 1)

A11 A/F Sensor

HTS

OXS

HAFR

AFR+

AFR–

HAFL

AFL+

AFL–

E10

E10

E7

E7

E7

E7

E7

E7

2B

W–B

B–R

J2
J/C

W

W

B–W

FL Block

A/F HTR

5

1

2

1

3

4

3

A

J/C

IA2

15

9

B

B

1

4

2

B–R

J3

J4

W

W

IA2

7

BR

IA2

EB1

3

1

4

2

4

3

1

2

B–R

L

R

G

BR

B–R

EB1

2

11

20

12

21

8

IH1

G

L

B–W

BR

BR–W

F11

R/B No. 2

+B

MT

E7

OX

W–B

–

DIAGNOSTICS

ENGINE

DI–43

LEXUS RX300   (RM785E)

HINT:

S

After confirming DTC P0125, use a hand-held tester to confirm voltage output of heated oxygen senĆ
sors (bank 1, 2 sensor 1) from CURRENT DATA.

S

The engine ECU controls the voltage of AFL+, AFR+ and AFL–, AFR– terminals of engine ECU to the
fixed voltage. Therefore, it is impossible to confirm the A/F sensor output voltage without a hand–held
tester.

WIRING DIAGRAM