Lexus RX300 (MCU15). Service manual — part 205

DI–52

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DIAGNOSTICS

ENGINE

LEXUS RX300 (RM785E)

DTC

P0171

System too Lean (Fuel Trim) (Bank 1)

DTC

P0172

System too Rich (Fuel Trim) (Bank 1)

DTC

P0174

System too Lean (Fuel Trim) (Bank 2)

DTC

P0175

System too Rich (Fuel Trim) (Bank 2)

CIRCUIT DESCRIPTION

Fuel trim refers to the feedback compensation value compared against the basic injection time. Fuel trim
includes short–term fuel trim and long–term fuel trim.
Short–term fuel trim is the short–term fuel compensation used to maintain the air–fuel ratio at its ideal
theoretical value.
The signal from the A/F sensor is approximately proportional to the existing air–fuel ratio, and engine ECU
comparing it with the ideal theoretical value, the engine ECU reduces fuel volume immediately if the air–fuel
ratio is rich and increases fuel volume if it is lean.
Long–term fuel trim compensates the deviation from the central value of the short–term fuel trim stored up
by each engine tolerance, and the deviation from the central value due to the passage of time and changes
of using environment.
If both the short–term fuel trim and long–term fuel trim exceed a certain value, it is detected as a malfunction
and the check engine warning light lights up.

DTC No.

DTC Detecting Condition

Trouble Area

P0171

P0174

When air fuel ratio feedback is stable after engine warming up,

fuel trim is considerably in error on RICH side

(2 trip detection logic)

S Gas leakage on exhaust system

S Air intake (hose loose)

S Fuel line pressure

S Injector blockage

S Mass air flow meter

S Engine coolant temp. sensor

S A/F sensors (bank 1, 2 sensor 1)

P0172

P0175

When air fuel ratio feedback is stable after engine warming up,

fuel trim is considerably in error on LEAN side

(2 trip detection logic)

S Gas leakage on exhaust system

S Fuel line pressure

S Injector leak, blockage

S Mass air flow meter

S Engine coolant temp. sensor

S A/F sensors (bank 1, 2 sensor 1)

DI84B–01

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DIAGNOSTICS

ENGINE

DI–53

LEXUS RX300 (RM785E)

HINT:

S

When DTC P0171 or P0174 is recorded, the actual air–fuel ratio is on the lean side. When DTC P0172
or P0175 is recorded, the actual air–fuel ratio is on the rich side.

S

If the vehicle runs out of fuel, the air–fuel ratio is lean and DTC P0171 or P0174 is recorded. The check
engine warning light then comes on.

S

If the total of the short–term fuel trim value and long–term fuel trim value is within ± 35 % (80°C (176°F)
or more), the system is functioning normally.

S

The A/F sensors (bank 1, 2 sensor 1) output voltage and the short–term fuel trim value can be read
using the hand–held tester.

S

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

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.

1

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

NG

Repair or replace.

OK

2

Check injector injection (See page FI–18).

NG

Replace injector.

OK

3

Check air flow meter and water temp. sensor (See pages FI–30 and FI–63).

NG

Repair or replace.

OK

DI–54

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DIAGNOSTICS

ENGINE

LEXUS RX300 (RM785E)

4

Check for spark and ignition (See page IG–1).

NG

Repair or replace.

OK

5

Check fuel pressure (See page FI–5).

NG

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

OK

6

Check gas leakage on exhaust system.

NG

Repair or replace.

OK

7

Check the output voltage A/F sensors (bank1, 2 sensor1).

PREPARATION:
(a)

Connect the hand–held tester to the DLC3.

(b)

Warm up the A/F sensors (bank1, 2 sensor1) with the engine speed at 2,500 rpm for approx. 90 sec.

CHECK:
Read voltage value of A/F sensors (bank1, 2 sensor1) on the screen of the hand–held tester when you per-
form all the following conditions.
HINT:
The voltage of AFL+, AFR+ terminal of engine ECU is fixed to 3.3 V and AFL–, AFR– terminal to 3.0 V. There-
fore, it is impossible to check the A/F sensors (bank1, 2 sensor1) output voltage at the terminals (AFL+,
AFR+ / AFL–, AFR–) of the engine ECU.

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DIAGNOSTICS

ENGINE

DI–55

LEXUS RX300 (RM785E)

OK:

Condition

A/F Sensor Voltage value

Engine idling

Engine racing

S Not remain at 3.30 V

S Not remain at 3 8 V or more

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

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

open and close

S Not remain at 3.8 V or more

S Not remain at 2.8 V or less

HINT:

S

During fuel enrichment, there is a case that the output voltage of A/F sensors (bank1, 2 sensor1) is
below 2.8 V, it is normal.

S

During fuel cut, there is a case that the output voltage of A/F sensors (bank1, 2 sensor1) is above 3.8
V, it is normal.

S

If the output voltage of A/F sensors (bank1, 2 sensor1) remains at 3.30 V even after performing all the
above conditions, A/F sensors (bank1, 2 sensor1) circuit may be open.

S

If the output voltage of A/F sensors (bank1, 2 sensor1) remains at 3.8 V or more, or 2.8 V or less even
after performing all the above conditions, A/F sensors (bank1, 2 sensor1) circuit may be short.

OK

Go to step 9.

NG

8

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

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

9

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

Go