Oxygen Sensor: Description and Operation
OXYGEN SENSOR:Oxygen Sensor:
- The oxygen sensor monitors the percentage of residual oxygen remaining in the exhaust-gas mixture. This sensor forms part of a closed-loop circuit designed to consistently maintain an optimal air/fuel mixture.
- The mixture ratio of atmospheric oxygen to fuel at which the catalytic converter transforms pollutants at maximum efficiency is lambda = 1 (stoichiometric mixture ratio).
- The engine-management system responds to changes in the composition of the exhaust gas by varying its control of numerous functions; these changes can also serve as the first indications of possible malfunctions.
OPERATION:
- 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 (Lamda = 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.
POSSIBLE CAUSES OF MALFUNCTION:
- If the voltage does not fluctuate, or the sensor responds too slowly (lazy), the following conditions are possible causes:
- Slits or holes in the oxygen sensor tip are plugged.
- Sensor was thermally overloaded.
- Sensor damaged via silicone contamination (various sprays).
- Sensor was lead poisoned with fuel containing lead. Check for lead content using a "PLUMBTESMO" test strip.
- Terminal resistance in the signal wire.
- Oxygen sensor too cold--oxygen sensor heater does not function and sensor does not reach ideal operating temperature.
- Oxygen sensor control switched off (fuel injection system malfunctions stored in DTC memory).
- Oxygen sensor damaged with electrical contact cleaner (or equivalent). The spray is drawn in through the signal wire via capillary action resulting from pressure differential and temperature fluctuation and ends up coating the sensor element and air passages causing reduced sensitivity or control failure.
CLOSED-LOOP OXYGEN SENSOR CONTROL WITH OBD II:
- Within the OBD II strategy an additional oxygen sensor (G1O8) was integrated which lies behind the catalytic converter. It serves only to check catalyst function.
- Depending upon the individual vehicle version, different connector plugs, colors or installation locations are employed to prevent the sensors from being connected incorrectly.
REQUIREMENTS:
- If the oxygen sensor mounted upstream from the catalytic converter is malfunctioning or damaged due to aging, it will become impossible to adjust the air/fuel mixture to its optimum ratio. The results are higher emissions and sacrifices in vehicle performance.
- It is thus essential that the engine-management system recognize these types of malfunctions when the DTC conditions are met, and that it reacts by storing the DTC code and activating the MIL.
OXYGEN SENSOR CLOSED-LOOP CONTROL:
- OBD II monitors the following parameters for oxygen sensor closed- loop control:
- Response characteristics and reaction to aging process.
- Voltage.
- Oxygen sensor heater.
OXYGEN SENSOR AGING DIAGNOSIS:
- Aging and contamination can affect the oxygen sensor's response characteristics. Reduced efficiency may be reflected in extended reaction times (periodicity) or a displacement of the sensor's voltage curve (sensor shift). Either of these will lead to contraction of the Lamda window, ultimately reducing the efficiency with which the catalytic converter carries out its conversion process.
- A change to the reaction time can be recorded, stored and displayed, however it can not be adjusted.
- Operation of the oxygen sensor is evaluated by checking the voltage that it generates. Short circuits (for instance, due to broken wires) or malfunction. A malfunction is determined, if a signal is too large or too small or recognized as malfunctioning.
OXYGEN SENSOR G39/G108:
Connector View - Oxygen Sensor G39/G108:
Electrical Circuit - Oxygen Sensor G39/G108:
- Oxygen sensor G39 is the Pre-Catalyst sensor of Bank 1, G108 is that of Bank 2.
Effects of Signal Failure:
- The system responds to failure to receive a signal from the oxygen sensor by cancelling the oxygen sensor control and suppressing the lambda adaptation function.
- The evaporative emissions control system reverts to emergency backup operation. The engine-management ECM operates according to a stored emergency program.
OXYGEN SENSOR G130/G131:
Connector View - Oxygen Sensor G130/G131:
Electrical Circuit - Oxygen Sensor G130/G131:
- Oxygen sensor G13O is the Post-Catalyst probe of Bank 1, G131 is that of Bank 2.
Effects of Signal Failure:
- Closed oxygen sensor control is maintained in the event of failure in the post-converter oxygen sensor. However, failure in the post-converter sensor makes it impossible to monitor operation of the catalytic converter. Catalyst function can not be checked if sensor malfunctions.
BENEFITS OF HEATED OXYGEN SENSORS:
Heated Oxygen Sensor - G39/G108:
Heated Oxygen Sensor - G130/G131:
- The oxygen sensor's response characteristics depend upon temperature. By heating the oxygen sensor it is possible to achieve effective closed-loop control of emissions at low engine and exhaust-gas temperatures.
DIAGNOSIS OF THE OXYGEN SENSOR'S HEATER:
- The system uses measurements of electrical resistance in the sensor's heater to verify that it is operating correctly.
- Condensation, the formation of which is a particular hazard in the cold-start phase, can cause damage to the heated oxygen sensor under unfavorable circumstances. Therefore the heating of the before-catalyst sensor is conducted directly after engine start, while the pre-catalyst sensor is heated after exceeding a calculated catalyst temperature.