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Lambda Sensor (Zirconia) Waveform Notes

The lambda sensor is also referred to as the Oxygen (O2) sensor or a Heated Exhaust Gas Oxygen (HEGO) sensor and plays a very important role in control of exhaust emissions on a catalytic equipped vehicle. The lambda sensor is fitted into the exhaust pipe before the catalytic converter.

The sensor will have varying electrical connections and may have up to four wires; it reacts to the oxygen content in the exhaust system and will produce a small voltage depending on the Air/Fuel mixture seen at the time. The voltage range seen will, in most cases, vary between 0.2 and 0.8 volts: 0.2 volts indicates a lean mixture and a voltage of 0.8 v shows a richer mixture.

A vehicle equipped with a lambda sensor is said to have ‘closed loop’, this means that after the fuel has been burnt during the combustion process, the sensor will analyse the emissions and re-adjust the engine’s fueling accordingly.

Lambda sensors can have a heater element to assist the sensor reaching its optimum operating temperature. Zirconia sensors when working correctly will switch approximately once per second (1 Hz) and will only start to switch when at normal operating temperature. This switching can be seen on the oscilloscope, and the waveform should look similar to the one in the example waveform. If the frequency of the switching is slower than anticipated, remove the sensor and clean with a solvent spray and this may improve the response time.