Further guidance
An oxygen sensor also can be referred to as a Lambda sensor, an O2 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 titania oxygen sensor has a titania element with a resistance dependent on O2 concentration. The resistance is low with low O2 concentrations (rich mixtures) and high with high O2 concentrations (lean mixtures). Therefore, given that the sensor is supplied with a 5 V reference, a rich mixture will cause a high voltage output, around 4.5 V, and a lean mixture will cause a low voltage output, around 0.2 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.
Titania oxygen sensors generally have four wires, as they do not generate their own voltage output: two wires will provide a 5 V reference and an earth circuit to the sensing element and another two wires will provide a feed and an earth circuit to the heating element.
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 airflow 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 supply, earth, signal, or heating circuits.
- Damage or contamination from excessive fuel in the exhaust.
- Damage from excessive heat.
- Incorrect fitment (and damage arising therefrom).
