Chapter 24 Sensor, Actuator, Computer Service 467
Figure 24-9 shows a scan tool reading of oxygen sen-
sor voltage. With an OBD II vehicle, this is the quickest
way of monitoring the voltage output from the O
2
sensors.
O
2
Sensor Circuit Testing
If the oxygen sensor has normal voltage, you should
check the circuit leading to the sensor. Measure the resis-
tance of the wires leading to the oxygen sensor. You can
use long test leads, as in Figure 24-7B. You can also ground
one end of the sensor wire and check it for continuity at the
other end, Figure 24-7C.
O
2
Sensor Replacement
Disconnect the negative battery cable. Then, separate the
sensor from the wiring harness by unplugging the connector.
Never pull on the wires themselves as damage may result.
Figure 24-7. Oxygen sensor is a common trouble point. It can be tested with a digital voltmeter. A—Connect the digital voltmeter to
the oxygen sensor and to ground. With the engine running and warmed, the sensor voltage should be around one-half volt. B—If the
sensor checks good, also check the wiring leading to the sensor. Wiring should have good continuity between the computer and the
sensor connector. C—Continuity good between the computer and sensor. (Ford)
~V
OFF
V
Ω
mA
mA
A
~
0 Ω
~V
OFF
V
Ω
mA
mA
A
~
0.5 V
Good
Continuity
Ground
Digital
Voltmeter
Engine Running
Oxygen Sensor
Lead
Body
Structure
Wire from
Computer to
O2 Sensor
O2 Sensor
Disconnected
To Computer
Disconnect
from Computer
Ohmmeter
Ohmmeter
Should Show
Little or
No Ohms
A B C
Figure 24-8. Note the typical operating voltages for an oxygen sensor. As the throttle is snapped open and closed to alter fuel meter-
ing, the oxygen sensor voltage should swing up and down to signal the computer or fuel mixture change.
1/2 Volt
Lean
Voltage Changes with Oxygen
in Engine Exhaust and Oxygen
Changes with Air-Fuel Ratio
16:1 15:1 14.7:1 14:1 13:1
Rich
Mixture
Lean
Mixture A/F Ratio
0 Volt
100
200
300
400
500 Voltage
in mV
600
700
800
900
1 Volt
Rich
O2 Sensor
Voltage
Band