Lexus RX300 (MCU15). Service manual — part 203

DI–44

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DIAGNOSTICS

ENGINE

LEXUS RX300 (RM785E)

INSPECTION PROCEDURE

HINT:
Read freeze frame data using a 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.

1

Are there any other codes (besides DTC P0125) being output ?

YES

Go to relevant DTC chart.

NO

2

Connect hand–held tester and read value for voltage output of A/F sensors
(bank 1, 2 sensor 1).

PREPARATION:
(a)

Connect the hand–held tester to the DLC3.

(b)

Warm up the A/F sensors (bank 1, 2 sensor 1) with the engine at 2,500 rpm for approx. 90 sec.

CHECK:
Read voltage value of A/F sensors (bank 1, 2 sensor 1) on the screen of hand–held tester when you perform
all the following conditions.
HINT:
The voltage of AFL+, AFR+ terminal of the engine ECU is fixed to 3.3 V and the AFL–, AFR– terminal to 3.0
V. Therefore, it is impossible to check the A/F sensor output voltage at the terminals (AFL+, AFR+/AFL–,
AFR–) of the engine ECU.
OK:

Condition

A/F Sensor Voltage value

Engine idling

Engine idling

Not remains at 3 3 V

Engine idling

S Not remains at 3.3. V

Driving at engine speed 1,500 rpm or more and vehicle

speed 40 km/h (25 mph) or more, and operate throttle valve

open and close

S Not remains at 3.8 V or more

S Not remains at 2.8 V or less

HINT:

S

During fuel enrichment, there is a case that the output voltage of A/F sensors (bank 1, 2 sensor 1) is
below 2.8 V

, it is n ormal.

S

During fuel cut, there is a case that the output voltage of A/F sensors (bank 1, 2 sensor 1) is above
3.8 V

, it is normal.

S

If output voltage of A/F sensors (bank 1, 2 sensor 1) remains at 3.30 V even after performing all the
above conditions, A/F sensors (bank 1, 2 sensor 1) circuit may be open.

S

If output voltage of A/F sensor remains at 3.8 V or more, or 2.8 V or less even after performing all the
above conditions, A/F sensors (bank 1, 2 sensor 1) circuit may be short.

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DIAGNOSTICS

ENGINE

DI–45

LEXUS RX300 (RM785E)

OK

Go to step 9.

NG

3

Check for open and short in harness and connector between engine ECU and
A/F sensors (bank1, 2 sensor1) (See page IN–32).

NG

Repair or replace harness or connector.

OK

4

Check resistance of A/F sensor heaters (bank1, 2 sensor1) (See page FI–67).

NG

Replace A/F sensor.

OK

5

Check air induction system (See page FI–1).

NG

Repair or replace.

OK

6

Check fuel pressure (See page FI–5).

NG

Check and repair fuel pump, fuel pipe line and
filter (See page FI–1).

OK

DI–46

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DIAGNOSTICS

ENGINE

LEXUS RX300 (RM785E)

7

Check injector injection (See page FI–18).

NG

Replace injector.

OK

8

Check gas leakage on exhaust system.

NG

Repair or replace

OK

Replace A/F sensors (bank1, 2 sensor1).

9

Perform confirmation driving pattern (See page DI–85).

Go

10

Is there DTC P0125 being output again ?

YES

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

NO

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DIAGNOSTICS

ENGINE

DI–47

LEXUS RX300 (RM785E)

11

Did vehicle run out of fuel in the past ?

NO

Check for intermittent problems
(See page DI–3).

YES

DTC P0125 is caused by running out of fuel.