My journey begins with a 2005 Nissan Altima. The vehicle was being inspected in preparation for emissions testing, and we have always used a common rule that if you are looking at good engine performance and emissions, the vehicle must be in fuel control. There are many vehicles where the oxygen sensor can be used as a common tool to embrace this concept. This was a somewhat easy task for zirconia dioxide oxygen sensors, but this vehicle had a different type of oxygen sensor. It is a Bosch LSU 4.9 wideband air/fuel ratio oxygen sensor (5 Wire). The first question that comes to mind is: How does this sensor work?
In 1997, Bosch developed a flat ceramic zirconia element rather than a thimble design for their oxygen sensor. The name "planar" was used because the sensor element is a flat strip of ceramic that is at or about 1.5mm in thickness. The new design works similar to a thimble-type zirconia dioxide sensor, but this "thick-film" design as it is called makes it smaller, lighter, and more resistant to contamination.
The new heater element also requires less electrical energy and brings the sensor element up to operating temperature in about 10 seconds. This new Bosch LSU 4.X wideband air/fuel ratio sensor combines the oxygen-sensing "Nernst" cell from the planar sensor with an "oxygen pump" to create a device that can actually measure air/fuel ratios.
The Nernst cell still senses oxygen in the same way that a conventional thimble-type O2 sensor does. When there's a difference in oxygen levels across the zirconium dioxide sensor element, current flows from one side to the other and produces a voltage (Figure 1) (Typical Voltage across Nernst Cell = .45V).
To get more precision, the oxygen pump uses a heated cathode and anode to pull some oxygen from the exhaust into a "diffusion" gap between the two components. The Nernst cell and oxygen pump are wired together in such a way that it takes a certain amount of current to maintain a balanced oxygen level in the diffusion gap. The amount of current required to maintain this balance is directly proportional to the oxygen level in the exhaust.
This gives the engine computer the precise air/fuel measurements it needs to meet the new emission requirements. To compensate, the computer adjusts the fuel mixture by adding more fuel when the mixture is lean or using less fuel when it is rich. This is the basics of feedback fuel control (Figure 2).
I put together a series of planned steps that would help me move forward in my understanding of this sensor. I laid out the steps as follows:
- Operation and Description
- Review Wiring Diagram (Trim Resistor Specification)
- Scan Data Analysis
- Voltage and Current Worksheet
1. Operation and description
I needed to seek an understanding of how this sensor worked. Here is the description that was available in reference to service information:
"The A/F sensor 1 is a planar dual-cell limit current sensor. The sensor element of the A/F sensor 1 is the combination of a Nernst concentration cell (sensor cell) with an oxygen-pump cell, which transports oxygen ions. It has a heater in the element. The sensor is capable of precise measurement Lambda = 1, but also in the lean and rich range. Together with its control electronics, the sensor outputs a clear, continuous signal throughout a wide Lambda range (0.7 < Lambda < air).
The exhaust gas components diffuse through the diffusion gap at the electrode of the oxygen pump and Nernst concentration cell, where they are brought to thermodynamic balance."
This provided very little understanding for me at first glance, I needed certain terms defined to assist in my understanding.
- Conventional Planar O2S (Narrow Band)
- Single Cell Design AFR Sensor: A single-cell O2 sensor looks similar to a narrowband 4-wire sensor, but they can be either a planar design or cup-style (thimble). The four wires designate two for the heater and two for the signal and operate at 0.4 volts for the sensor.
- Planar Dual Cell Limit Sensor: Dual-cell AFR sensors use a planar design with a flat zirconia dioxide element that is about 1.5mm thick. The planar design has several advantages over a single-cell AFR sensor due to its quicker warmup times, faster switching times and better resistance to contamination.
- Nernst Concentration Cell: In terms of design, the wide band oxygen sensor is similar to a conventional planar O2S in reference to the area labeled the Nernst Cell. The Nernst cell for a wide band sensor senses oxygen in the same way that a conventional thimble type O2 sensor does. When there is a difference in oxygen levels across the zirconium sensor element, current flows from one side to the other and produces a voltage, but this gives only a gross rich-lean indication of the air/fuel mixture. The goal is to maintain a voltage of .45V across the Nernst cell.
- Pump Cell (Pump Current): Above the Nernst Concentration Cell is another zirconia layer with two electrodes, which is called the pump cell. The two cells share a common ground, which is called the reference. There are two internal chambers:
- Air Reference chamber (exposed to ambient air)
- Diffusion Gap or Chamber (exposed to exhaust gases)
When the exhaust is rich, the PCM applies a negative current to the pump cell. When the exhaust is lean, the PCM applies a positive current to the pump cell.
2. Review wiring diagram (Trim Resistor Specification)
Now that I understand the sensor arrangement, and the roles of the Nernst and Pump cells a little better, it's time to move on to Step No. 2. Before installing a new sensor, it may not be a bad idea to check the resistance of the trim resistor, Bosch indicates a range of 30-300 ohms (Figures 3 and 4). It is important to note, though, that the resistance for the trim resistor on any two sensors may not be the same but yet still be within specifications.
Test Points (Ohmmeter) for testing the trim resistor calibration is shown in Figure 5, cover removed. (30-300 Ohms).
The wiring diagram in Figure 6 is typical of what you may see in service information, please note that there are six wires shown in the diagram. There will be a total of six wires on the ECM side of the circuit but only five wires on the actual Bosch wide range oxygen sensor. If you look closely at the Bosch wiring diagram above and follow the trim resistor circuit, it will offer some insight.
After careful review and some homework, I was able to obtain information on the meaning of the designations listed on the wiring diagram. This was very important in reference to knowing what voltage information is present at each terminal:
- AF-H1 – Heater control
- A/F IA1 – Pumping Cell via Trim Resistor
- AF-VM1 – Reference (common) 2.5V above PCM ground
- AF-UN1 – Nernst Cell Voltage (450 mv when measured from AF-UN1 to AF-VM1 - also carries current for pumped reference)
- AF-IP1 – Pumping Cell
- Fused Circuit- Heater feed to sensor from battery
3. Scan Data Analysis
This information from Nissan implies that we should see about 1.5V (PCM Produced Voltage) on the scantool KOEO(Figures 7, 8, and 9) will support this. Nissan uses a concept called Alpha (correction). Here is how to interpret Alpha:
- Alpha = 100 (No Correction Needed)
- Alpha > 100 (PCM is adding fuel because it thinks the engine is lean)
- Alpha < 100 (PCM is subtracting fuel because it thinks the engine is rich)
Step 4: Oxygen Sensor Testing (Voltage and Current Worksheet)
This information from Nissan does not provide terminal information as shown Figure 10, and it makes the understanding somewhat difficult in terms of diagnostics. I have provided some important notes to help understand what the technician would see voltage wise at these noted terminals. I found that using a worksheet helps in gathering the data. The measurements are made from the noted terminal to battery negative (Figure 11) (Please note that wire colors for these sensors can and will vary).
Figures 12 and 13 show the voltage and current for the oxygen sensor heater element, the voltage level peaks at 11.6V, and the current peaks at 1.2A. There is a nice clean transition in terms of pulse width modulation.
Figure 13 shows current measurement for pump current (milliamps), I am using a low amperage probe that can measured these small currents. When propane was added, the current went negative and when propane was taken away the current went positive. This was a great example of a working sensor. This is my 4-step process for the Bosch 5 wire sensors, I apply this same process to their 4-wire design.
