check engine AUDI A4 2000 B5 / 1.G AFC Engine Oxygen Sensor Control Checking Workshop Manual

24-41
Oxygen Sensor (O2S) control, checking Notes:

Do not use sealants containing silicone to seal the intake area on engines with oxygen sensors. Silicone particles are not
consumed during combustion, consequently the unburned particles travel via the exhaust stream to the oxygen sensor
where they can eventually coat the sensor probe and interrupt or destroy proper oxygen sensor function. Do not use
electrical contact cleaner (or equivalent) in the area of the heated oxygen sensor harness connector because it can lead to
corrosion damage of the oxygen sensor.
The heated oxygen sensor compares the oxygen content of the "outside" air with the quantity of oxygen in the exhaust
stream and generates a corresponding voltage signal as an input to the Engine Control Module (ECM). The "mixture too
rich" voltage signal (low oxygen content) is approx. 0.6 to 0.9 volts. The "mixture too lean" voltage signal (high oxygen
content) is about 0.0 to 0.3 volts. During the transition from "rich" to "lean" (or the reverse) the voltage fluctuates between
0.6 to 0.9 volts and 0.0 to 0.3 volts.
Due to the abrupt voltage fluctuation, oxygen sensor output can correspond to the ideal or "stoichiometric" mixture ( =
1.0), but mixture control is not held constant to this value; instead, the control constantly fluctuates back and forth in a
window between the "mixture too lean" and "mixture too rich" conditions. For a warm engine, the control frequency during
idle is 0.5 Hz. minimum (30 cycles per minute). With increased engine speed (2500 to 2800 RPM) the control frequency
must be at least 1Hz.(60 cycles per minute). One cycle means a voltage swing from the highest value to the lowest value
and back again to the highest.
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