Toyota Sequoia (2005). Manual - part 147

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DIAGNOSTICS

ENGINE

DI–383

577

DTC

P2195

Oxygen (A/F) Sensor Signal Stuck Lean
(Bank 1 Sensor 1)

DTC

P2196

Oxygen (A/F) Sensor Signal Stuck Rich
(Bank 1 Sensor 1)

DTC

P2197

Oxygen (A/F) Sensor Signal Stuck Lean
(Bank 2 Sensor 1)

DTC

P2198

Oxygen (A/F) Sensor Signal Stuck Rich
(Bank 2 Sensor 1)

HINT:

Although the DTC titles say oxygen sensor, these DTCs relate to the Air–Fuel Ratio (A/F) sensor.

Sensor 1 refers to the sensor mounted in front of the Three–Way Catalytic Converter (TWC) and lo-
cated near the engine assembly.

DIDFT–01

A23473

A–A Cross Section

Atmospheric Air

Solid Electrolyte

(Zirconia Element)

Platinum 
Electrode

Heater

Exhaust Gas

Cover

Element

A

A

ECM Monitored

A/F Sensor V

oltage

Air–Fuel Ratio

Alumina

4.0

3.0

2.0

12

13

14

15

16

17

DI–384

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DIAGNOSTICS

ENGINE

578

CIRCUIT DESCRIPTION

The A/F sensor generates a voltage* that corresponds to the actual air–fuel ratio. This sensor voltage is used
to provide the ECM with feedback so that it can control the air–fuel ratio. The ECM determines the deviation
from the stoichiometric air–fuel ratio level, and regulates the fuel injection time. If the A/F sensor malfunc-
tions, the ECM is unable to control the air–fuel ratio accurately.
The A/F sensor is the planar type and is integrated with the heater, which heats the solid electrolyte (zirconia
element). This heater is controlled by the ECM. When the intake air volume is low (the exhaust gas tempera-
ture is low), a current flows into the heater to heat the sensor, in order to facilitate accurate air–fuel ratio
detection. In addition, the sensor and heater portions are narrower than the conventional type. The heat
generated by the heater is conducted to the solid electrolyte though the alumina, therefore the sensor activa-
tion is accelerated. 
In order to obtain a high purification rate of the carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide
(NOx) components in the exhaust gas, a TWC is used. For the most efficient use of the TWC, the air–fuel
ratio must be precisely controlled so that it is always close to the stoichiometric level.
*: Value changes inside the ECM. Since the A/F sensor is the current output element, a current is converted
to a voltage inside the ECM. Any measurements taken at the A/F sensor or ECM connectors will show a
constant voltage.

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DIAGNOSTICS

ENGINE

DI–385

579

DTC No.

DTC Detection Conditions

Trouble Areas

P2195
P2197

While fuel–cut operation performing (during vehicle decelera-

tion), air–fuel ratio sensor current is 3.6 mA or more for 3 se-

conds.

A/F sensor (sensor 1)

ECM

P2195

Conditions (a) and (b) continue for 2 seconds or more 
(2 trip detection logic):

(a) Air–Fuel Ratio (A/F) sensor voltage more than 3.8 V

(b) Heated Oxygen (HO2) sensor voltage 0.15 V or more

Open or short in A/F sensor (sensor 1) circuit

A/F sensor (sensor 1)

A/F sensor (sensor 1) heater

EFI relay

A/F sensor heater and relay circuits

Air induction system

Fuel pressure

Injector

ECM

P2196
P2198

While fuel–cut operation performing (during vehicle decelera-

tion), air–fuel ratio sensor current is less than 1.4 mA for 3

seconds.

A/F sensor (sensor 1)

ECM

P2196

Conditions (a) and (b) continue for 2 seconds or more 
(2 trip detection logic):

(a) A/F sensor voltage less than 2.8 V

(b) HO2 sensor voltage less than 0.85 V

Open or short in A/F sensor (sensor 1) circuit

A/F sensor (sensor 1)

A/F sensor (sensor 1) heater

EFI relay

A/F sensor heater and relay circuits

Air induction system

Fuel pressure

Injector

ECM

HINT:

When any of these DTCs are set, check the A/F sensor voltage output by selecting the following menu
items on a hand−held tester: DIAGNOSIS / ENHANCED OBD II / DATA LIST / ALL / AFS B1S1.

Short−term fuel trim values can also be read using a hand−held tester.

The ECM regulates the voltage at the A1A+ and A1A– terminals of the ECM at a constant level. There-
fore, the A/F sensor voltage output cannot be confirmed without using a hand–held tester.

If the A/F sensor functional malfunction is detected, the ECM sets this DTC.

A23456

Air–fuel Ratio Sensor Current Monitor:

Fuel–cut

High Side Thresh

Low Side Thresh

3 sec. or more

Time

t2

t1

3 sec.
Delay

ON

OFF

Normal

Malfunction

3 sec.
Delay

3 sec.
Delay

Sensor Current
                 (mA)

A/F Sensor
Current

Cumulative Time = t1 + t2 = 3 sec. or more

DI–386

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DIAGNOSTICS

ENGINE

580

MONITOR DESCRIPTION

Sensor voltage detection monitor:
Under the air–fuel ratio feedback control, if the A/F sensor voltage output indicates rich or lean for a
certain period of time, the ECM determines that there is a malfunction in the A/F sensor. The ECM illu-
minates the MIL and sets a DTC.
Example:
If the A/F sensor voltage output is less than 2.8 V (very rich condition) for 10 seconds, despite the HO2
sensor voltage output being less than 0.85 V, the ECM sets DTC P2196. Alternatively, if the A/F sensor
voltage output is more than 3.8 V (very lean condition) for 10 seconds, despite the HO2 sensor voltage
output being 0.15 V or more, DTC P2195 or P2197 is set.

Sensor current detection monitor:
A rich air–fuel mixture causes a low air–fuel ratio sensor current, and a lean air fuel mixture causes
a high air fuel ratio sensor current. Therefore, the sensor output becomes high during acceleration,
and the sensor becomes low during deceleration.
The ECM monitors the air–fuel ratio sensor current during fuel–cut and detects an unusual current val-
ue. If the cumulative time the sensor output is out of range exceeds more than 3 seconds, the ECM
interprets a malfunction in the air–fuel ratio sensor and sets a DTC.