1. Oxygen Sensor Operation
Air Fuel (AF)
The AF sensor, also called a broadband planar sensor or Lean Air
Fuel sensor (LAF), used in some applications resembles the heated
Zirconia sensor in appearance only. Manufacturers changed to A/F
sensors because they are more accurate and have a wider range
than previous O2 sensors. Also A/F sensors can be operational
within 20 seconds which allows the PCM to control emissions
during warm-up.
The AF sensor improves overall efficiency by keeping the fuel
control system in closed-loop during a much wider range of
driving conditions. Subsequently, instead of using
preprogrammed, open loop air/fuel ratios in many situations, the
PCM fine-tunes the mixture much more closely based on the actual
oxygen readings from the exhaust.

2. Oxygen Sensor Operation
Air Fuel (AF)
AF sensors may be configured as seven-wire, five wire, or four-wire
sensors. AF sensors cannot be visually identified by the number of wires
they have. Always look at the under hood emissions label to determine
if a vehicle is equipped with an Air/Fuel sensor.

3. Oxygen Sensor Operation
Air Fuel (AF)
AF sensors are used as the pre-catalyst (upstream) oxygen sensors.
On a four-wire AF sensor, two wires are power and ground for the sensor
heater, and the other two wires are used for the exhaust mixture signal.

4. Oxygen Sensor Operation
Air Fuel (AF)
The Air Fuel oxygen sensor heater is designed to heat the oxygen sensor
thimble to a minimum of 1200 F. This temperature is double that of an
early four-wire sensor and is required for the AF sensor to properly
sample the exhaust oxygen content.
When the AF sensor heater is commanded on by the PCM, approximately
8 amps of current should be flowing through the circuit.

5. Oxygen Sensor Operation
Air Fuel (AF)
The PCM controls the voltage to a fixed voltage. It is difficult to confirm
the AF sensor voltage without a scan tool as the voltages at the terminals
are fixed and any change is noted within the PCM itself.
The voltage signal is proportional to the change in the air/fuel mixture.
This allows the PCM to more accurately judge the exact air/fuel ratio
under a wide variety of conditions and quickly adjust the amount of
injector pulse.
Think of the AF sensor as a generator that is capable of changing polarity.

6. Oxygen Sensor Operation
Air Fuel (AF)
The AF sensor has two zirconia elements that share a diffusion chamber.
The AF sensor is really two O2 sensors in one unit.
There are three chambers in the AF sensor:
• The first chamber contacts the exhaust flow.
• The diffusion chamber between the elements.
• The air reference chamber.

7. Oxygen Sensor Operation
Air Fuel (AF)

8. Oxygen Sensor Operation
Air Fuel (AF)
The first chamber is really the outside of the sensor, which contacts the
exhaust. The diffusion chamber is the area between the two zirconia
elements and the air reference chamber is at the other end. The basic
operating principle behind the AF sensor is that by controlling the amount
of O2 in the diffusion chamber, you can control its operating range.
One of the zirconia elements acts as an oxygen pump. We discussed how a
flow of oxygen ions creates a flow of electrons. The inverse is also true. A
flow of electrons applied to the sensor causes a flow of ions. This is what
happens when you charge a car battery.

9. Oxygen Sensor Operation
Air Fuel (AF)
Notice how the elements are wired in parallel, and there is a common
ground lead. This ground is a reference point for the ECM.
Do not confuse it with the vehicle ground. In fact, if you measure the
voltage between the sensor ground and the vehicle ground, you will see
about 2.7 volts.

10. Oxygen Sensor Operation
Air Fuel (AF)
For this explanation, we will distinguish the two zirconia elements by
calling one, sensor #1 and the other, sensor #2.
The ECM monitors the voltage between sensor #1 input and the ground
lead. The ECM tries to hold the voltage difference between sensor #1
and the ground lead to 450 mV.

11. Oxygen Sensor Operation
Air Fuel (AF)

12. Oxygen Sensor Operation
Air Fuel (AF)
When the mixture goes rich, oxygen ions flow from the diffusion
chamber to the exhaust. The voltage on sensor #1 input increases. The
ECM detects the voltage increase and reduces the voltage on sensor #2
input.

13. Oxygen Sensor Operation
Air Fuel (AF)
The voltage on sensor #2 input then goes more negative than the ground
voltage. This causes sensor #2 to pump oxygen out of the diffusion
chamber into the air reference chamber. When the oxygen content of the
diffusion chamber drops, the voltage on sensor #1 drops. At the same
time that the ECM reduces the voltage on sensor #2 input, it is also
reducing fuel delivery.

14. Oxygen Sensor Operation
Air Fuel (AF)
When the mixture goes lean, oxygen ions flow from the exhaust into the
diffusion chamber. The voltage on sensor #1 input decreases. The ECM
detects the voltage decrease and increases the voltage on sensor #2 input.
The voltage on sensor #2 input goes more positive than the ground
voltage. This causes sensor #2 to pump oxygen into the diffusion
chamber from the air reference chamber. The voltage between sensor #1
input and ground is consistently held at 450 mV.

15. Oxygen Sensor Operation
Air Fuel (AF)
The PCM knows how rich or lean the exhaust is by how much
amperage it supplies to sensor #2 input to hold sensor #1 input voltage
to 450 mV.
• Positive amperage the mixture is lean.
• Negative amperage the mixture is rich.

16. Oxygen Sensor Operation
Air Fuel (AF)
Rich Mixture
Lean Mixture
Zero Volts

17. Please wait for video to load and play.