Oxygen Sensors. Checking - BMW 5251 Service Manual

130-51
Fuel
Injection
Oxygen sensors, checking
The oxygen sensor provides an input voltage signal [0.2 vdc
(lean) to 0.8 vdc (rich)] to the ECM based on the oxygen con-
tent in the exhaust gas. T o generate voltage, the sensor tem-
perature must exceed 575" F (300" C). Therefore the sensor
is electrically heated.
NO JE
-
The test given below is not a conclusive test of oxygen sen-
sor efficiency and does not test how the oxygen sensor re-
acts to changing conditions. Pin numbers and wire colors
can vary Always check the wiring diagrams to confirm wire
color and pin assignment.
0
The oxygen sensorconnectorplugs are locatednext to the
catalytic converters.
With connector attached to sensor, connect digital voltmeter
between A and B wires in connector.
WARNING
-
Exhaust manifolds andpipes can be hot enough to cause se-
rious burns. Wear suitable heavy gloves and other appropri-
ate protection.
NOJE-
Connector style may vary depending on system application.
Consult wiring diagram for system-specific wiring. See ELE
Electrical Wiring Diagrams.
Start engine. A fluctuating voltage signal should show on the
meter. If voltage is incorrect, turn engine off and check pre-
heater circuit as described below.
NOJE-
To check sensor response to lean and rich mixtures, create
an air leak.
Oxygen sensor voltage at idle
-
Connect digital voltmeterbetween sensor heaterwires (white
wires, C and D). Start engine and check for battery voltage.
If voltage is not present, check oxygen sensor heater fuses.
0.2
to
0.8
vdc fluctuating
NOJE-
The oxygen sensor heater is powered via fusedpositive (+)
battery voltage and a switchedground from the ECM. See
ELE Electrical Wiring Diagrams for specific circuit informa-
tion.
-
The ground side of the oxygen sensor heater circuit is pulsed
on and off by the D M E control module.