Plug a BNC test lead into Channel A on the PicoScope, place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe onto the test lead with the red moulding (positive).
Place the black clip onto the battery negative terminal and probe the lambda sensors output connection with the Back-pinning Probe or multimeter probe as illustrated in Figure 1. Regardless of the number of wires connecting the lambda sensor to the vehicle's ECM, the output from the sensor will almost invariably be on the black wire. This connection is illustrated in Figure 1 on a single wire sensor.
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; cars using the new EOBD2 will also have a post cat lambda sensor.
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 resultant emissions and re adjust the engine's fuelling accordingly. lambda sensors can have a heater element which heats the sensor to its optimum operating temperature of 600C, this enables the sensor to be located further away from the heat source at the manifold to a 'cleaner' location. The sensor is inoperative below 300C.
The lambda sensor is essentially two porous platinum electrodes. The outer electrode surface is exposed to the exhaust gasses and is coated in a porous ceramic with the inner coated surface exposed to fresh air.
The most commonly used sensor uses a Zirconia element, producing a voltage when a difference in oxygen content is seen between the two electrodes. This signal is then sent to the Electronic Control Module (ECM) and the mixture is adjusted accordingly.
Titania is also used in the manufacture of another type of lambda sensor that offers a faster switching time than the more common Zirconia sensor.
The Titania Oxygen sensor differs from the Zirconia sensor in the fact that it is incapable of producing its own output voltage and is therefore reliant upon a 5 volt supply from the vehicle's ECM. The reference voltage is altered according to the engines air fuel ratio, with a lean mixture returning a voltage as low as 0.4 volt to a rich mixture producing a voltage in the region of 4.0 volts.
An ECM will only control the fuelling in 'closed loop' when the appropriate conditions allow, this is normally during: idle, light load and cruise operations. When the vehicle accelerates the ECM allows over fuelling and ignores the lambda signals. This is also the case for initial warm-up.
Both Titania and Zirconia sensors when working correctly will switch approximately once per second (1 Hz) and will both only start to switch once the normal operating temperature has been achieved. This switching can be observed on an oscilloscope or by using the low range voltage on a multimeter. When using an oscilloscope the resultant waveform should look as the illustration below. If the frequency of the switching is slower than anticipated, the removal of the sensor and cleaning with a solvent spray may improve the response time.
A constant high voltage output from the Zirconia shows that the engine is running constantly rich and is outside the ECM's adjusting range, where as a low voltage indicates a lean or weak mixture.
Figure 2 shows an example two wire Zirconia lambda sensor.
Single wire: this wire is the self generated voltage output from the sensor and is generally black in colour.
Two wire: this will have an output wire and an output earth return.
Three wire: this will have a single output wire and two wires for the heater element (supply and earth). The internal heating element raises the temperature to ensure faster control when starting from cold.
Four wire: this unit has signal and signal earth return wires. The additional two wires are for the heater element.
Typical wiring arrangements for both types of sensor are shown in Figure 3.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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