Oxygen sensor (zirconia)

The purpose of this test is to evaluate the operation of a zirconia type oxygen sensor during engine running conditions based on its output voltage and response time.

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the sensor signal circuit.
2. Connect PicoScope Channel A to the sensor signal circuit.
3. Allow the engine to run at idle speed until normal operating temperature is reached.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope to see live data.
6. With your waveform on screen stop the scope.
7. Turn off the engine.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• A cyclic voltage output varying between a low value, around 0.25 V, and a high value, around 0.7 V.
• When they occur, the transitions between the low and high voltages are quick and generally take no longer than 0.5 s.
• The voltage peaks appear at around 1 s intervals giving the waveform a frequency of 1 Hz.
• The waveform is fairly uniform with no drop-out or irregular anomalies.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Oxygen / O2/ Lambda sensor.

Further guidance

An oxygen sensor also can be referred to as a Lambda sensor, an O_­2 sensor, or a Heated Exhaust Gas Oxygen (HEGO) sensor. It is a feedback sensor used by the Engine Control Module (ECM) to perform closed loop control of engine fueling and, if a post catalytic converter sensor is present, monitor catalytic converter function.

Closed-loop control allows an ECM to maintain an almost exactly stoichiometric air/fuel mixture but with small variations between slightly rich and slightly lean to facilitate 3-way catalytic converter operation. These fueling variations cause the switching observed in the sensor voltage output. Typically, an ECM will switch the air/fuel ratio at a frequency around 1 cycle per second.

An ECM only undertakes closed loop control of fueling when the appropriate conditions allow. This is normally during steady-state idle, light load, or cruise operations. When the engine systems are warming up or the vehicle accelerates, the mixture is enriched and the sensors will not exhibit their switching output behaviour.

A zirconia element within the sensor allows ionized O_­2 to flow from a reference air source to the exhaust gas. The flow is detected by two platinum electrodes either side of the element. The flow rate depends on the partial pressure (the relative O_­2 concentrations within the reference air source and exhaust gas). A rich mixture will cause a greater flow of ionized O_­2 across the zirconia element, whereas a lean mixture will cause a low flow. Therefore, a lean mixture is indicated by a low output voltage, around 0.2 V, whereas a rich mixture is indicated by a high output voltage, around 0.8 V.

In general, oxygen sensors will not operate below around 300 °C. As such, some sensors have an internal heating element, which is controlled by the ECM. The heating element raises the temperature to ensure faster control when starting from cold.

Sensor configurations (zirconia type only)

The sensors have varying electrical configurations and may have up to four wires. Sensors without heating elements have only one or two wires. A three wire sensor uses the sensor casing to form an earth connection for the sensor element:

• A single wire providing a sensor output circuit.
• Two wires providing sensor output and earth circuits.
• Three wires providing a sensor output circuit, as well as heating element supply and earth circuits.
• Four wires providing sensor output and earth circuits, as well as heating element supply and earth circuits.

A constant high voltage output from the sensor shows that the engine is running constantly rich and is outside the ECM's adjusting range, whereas a constant low voltage indicates a lean or weak mixture. Under these conditions you can expect to see Diagnostic Trouble Codes (DTCs) related to fuel trim issues from the ECM. The sensor may not be at fault and you should ensure there are no associated issues causing the error codes before condemning the sensor.

Symptoms of a faulty/inoperative oxygen sensor:

• Malfunction Indicator Lamp (MIL) illumination.
• Diagnostic Trouble Codes (DTCs).
• No ECM switching between lean and rich mixtures (for catalytic converter operation).
• Fuel trim related faults.
• Smell of fuel vapours.
• Random multiple misfires.
• Driveability issues.
• Performance issues.

Related issues that must be eliminated prior to testing an oxygen sensor:

• Intake air leaks.
• Exhaust leaks.
• Blocked air intake or exhaust.
• Engine mechanical problems (including valve timing) causing incorrect air flow through the engine.
• Load sensors (e.g. air mass meter or manifold absolute pressure sensors) faults.
• Injection system faults causing over or under fueling.
• Ignition faults causing misfires.

Typical issues and oxygen sensor faults:

• Excessive fouling causing a slower, diminished or lack of response.
• Open or short circuits or high resistances in the sensor circuits, such as in the:
  • Sensor signal circuit.
  • Sensor supply voltage.
  • Sensor earth.
  • Sensor heating circuit.
• Damage or contamination from excessive fuel in the exhaust.
• Damage from excessive heat.
• Incorrect fitment (and damage arising therefrom).

GT022-EN