Toyota Sequoia (2005). Manual - part 90

A19288

EFI Relay

–

DIAGNOSTICS

ENGINE

DI–163

357

16

Check EFI relay.

PREPARATION:
Remove the EFI relay from the engine room J/B.
CHECK:
Inspect the EFI relay.
OK:

Standard:

Terminal No.

Condition

Specified Condition

3 – 5

Always

10 K

Ω

 or higher

3 – 5

Apply B+ between 

terminals 1 and 2

Below 1  

Ω

NG

Replace EFI relay.

OK

A23543

+B

OXR2
OXL2

E1

HTR2
HTL2

H2

H4

Wire Harness Side:

Bank 1 Sensor 2, Bank 2 Sensor 2

B17416

HT1B

ECM Connector

E7

OX1B

E8

HT2B

OX2B

A21040

Reference (Bank 1 Sensor 1 System Drawing)

From 
Battery

EFI No. 1 Fuse

EFI Relay

EFI No. 2 Fuse

Heater Oxygen
Sensor

Heater

Sensor

HT1A

OX1A

E2

MREL

ECM

DI–164

–

DIAGNOSTICS

ENGINE

358

17

Check for open and short in harness and connector between ECM and heated
oxygen sensor.

PREPARATION:
(a)

Disconnect the heated oxygen sensor connector.

(b)

Disconnect the E7 and E8 ECM connector.

CHECK:
Measure the resistance between the wire harness side connec-
tors.
OK:

Standard:

Tester Connection

Specified Condition

OXL2 (H2–3) – OX1B (E8–18)

Below 1 

Ω

HTL2 (H2–1) – HT1B (E8–1)

Below 1 

Ω

OXR2 (H4–3) – OX2B (E7–33)

Below 1 

Ω

HTR2 (H4–1) – HT2B (E7–5)

Below 1 

Ω

OXL2 (H2–3) or OX1B (E8–18) – 

Body ground

10 k

Ω

 or higher

HTL2 (H2–1) or HT1B (E8–1) – 

Body ground

10 k

Ω

 or higher

OXR2 (H4–3) or OX2B (E7–33) – 

Body ground

10 k

Ω

 or higher

HTR2 (H4–1) or HT2B (E7–5) – 

Body ground

10 k

Ω

 or higher

NG

Repair or replace harness or connector.

OK

Replace heated oxygen sensor.

–

DIAGNOSTICS

ENGINE

DI–165

359

DTC

P0171

System too Lean (Bank 1)

DTC

P0172

System too Rich (Bank 1)

DTC

P0174

System too Lean (Bank 2)

DTC

P0175

System too Rich (Bank 2)

CIRCUIT DESCRIPTION

The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim con-
sists of both the short–term and long–term fuel trims.
The short–term fuel trim is fuel compensation that is used to constantly maintain the air–fuel ratio at stoichio-
metric levels. The signal from the Air–Fuel Ratio (A/F) sensor indicates whether the air–fuel ratio is rich or
lean compared to the stoichiometric ratio. This triggers a reduction in the fuel injection volume if the air–fuel
ratio is rich and an increase in the fuel injection volume if it is lean.
Factors such as individual engine differences, wear over time and changes in operating environment cause
short–term fuel trim to vary from the central value. The long–term fuel trim, which controls overall fuel com-
pensation, compensates for long–term deviations in the fuel trim from the central value caused by the short–
term fuel trim compensation.
If both the short–term and long–term fuel trims are lean or rich beyond predetermined values, it is interpreted
as a malfunction, and the ECM illuminates the MIL and sets a DTC.

DIDFQ–01

DI–166

–

DIAGNOSTICS

ENGINE

360

DTC No.

DTC Detecting Condition

Trouble Area

P0171

P0174

When air–fuel ratio feedback is stable after warming up the

engine, fuel trim is considered to be in error on LEAN side 

(2 trip detection logic)

Air induction system

Injector blockage

Mass air flow meter

Engine coolant temperature sensor

Fuel pressure

Gas leakage in exhaust system

Open or short in heated oxygen sensor (bank 1, 2 sensor 1)

circuit

Heated oxygen sensor (bank 1, 2 sensor 1)

Heated oxygen sensor heater (bank 1, 2 sensor 1)

EFI relay

PCV piping

ECM

P0172

P0175

When air–fuel ratio feedback is stable after warming up the

engine, fuel trim is considered to be in error on RICH side 

(2 trip detection logic)

Injector leak, blockage

Mass air flow meter

Engine coolant temperature sensor

Ignition system

Fuel pressure

Gas leakage in exhaust system

Open or short in heated oxygen sensor (bank 1, 2 sensor 1)

circuit

Heated oxygen sensor (bank 1, 2 sensor 1)

ECM

HINT:

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.

If the vehicle runs out of fuel, the air–fuel ratio is LEAN and DTC P0171 or P0174 may be recorded.
The MIL then comes on.

If the total of the short–term fuel trim value and long–term fuel trim value is within 

 35 % (engine cool-

ant temperature is more than 75 



C (167



F)), the system is functioning normally.