# Oxygen sensor

The oxygen sensor is installed in the exhaust stream and measures the the oxygen content in the vehicle exhaust. By analyzing the voltage waveform from the oxygen sensor it is possible to determine if the sensor itself is operating as designed. Additionally some aspects of the engine management system can be diagnosed and verified as well. Faulty oxygen sensors can cause poor fuel mileage, increased emissions, poor engine performance and unstable idle. Additionally, emissions control devices such as the catalytic converter can be damaged or operate with reduced efficiency. Oxygen sensors compares the oxygen level in the exhaust with the level in the ambient air and presents the difference as an analog signal. Narrow band oxygen sensors use zirconium or titanium oxide and have the largest sensitivity at stoichiometric air/fuel ratios (Lambda=1). The Zirconium type acts as a galvanic battery and puts out a voltage whereas the Titanium oxide acts like a variable resistor. Titanium type sensors are very rare, they were installed on less than 1% of all vehicles in the late nineties.

Oxygen sensors using zirconium oxide.
A good Zirconium type oxygen sensor will put out a voltage that is usually between 0,2-0,9V. The voltage at Lambda=1 is 0,45V.

The waveform from a Bosch Zirconium oxygen sensor. A: – This is the voltage from the oxygen sensor at the cursor. The voltage is 840 mV, which is acceptable; A-B: – This is the voltage difference between the two cursors and is equal to 740 mV, which is also acceptable.

When the oxygen content in the exhaust is low (rich air/fuel mixture), the voltage is high (on the order of 0.65-1V). When the oxygen content in the exhaust is high (lean air/fuel ratio), the voltage is low (on the order of 40-250mV).
The sensing element must reach a voltage of about ~350°Ñ. The sensing element act as a very large value resistor until it reaches operating temperature. The engine management computer is often designed to impress a voltage of some value (often 450mV, the stoichiometric voltage) on the sensor through a large value resistor. The control unit does not consider the oxygen sensor signal to be valid until the sensor has the ability to reject this voltage (change it by more than ±150~250mV).

This waveform shows oxygen sensor activity as the sensor reaches operating temperature.
dT: – value of an interval of time between the two cursors. In this case it is the warm-up time of the Oxygen sensor and it is equal to 30 seconds;
A: – This is the voltage value at the cursor and is equal to the voltage impressed by the engine management control unit ~450mV;
A-B: – This is the difference voltage between the two cursors and is equal to the oxygen sensors ability to modify or reject the voltage impressed by the engine controller. This value is equal to ~250mV and the oxygen sensor signal is accepted by the control unit.

It should be noted that some engine management control units may use a different voltage value. DaimlerChrysler, for example, often use 5V.
Because the sensor does not work properly until it reaches operating temperature, it is often equipped with a heating element.

The wiring diagram for the connection of the Bosch Zirconium oxide Oxygen sensor. 1 – The connection point for obtaining waveforms from the oxygen sensor.

The engine controller uses the voltage signal from the oxygen sensor to control the air/fuel ratio within a 2-3% range. However, due to the distance from the exhaust valve(s) and the inherent delay in the sensor itself, the air/fuel ratio will dither or oscillate around a mean value. In most cases the mean value will be close to Lambda=1.

Oxygen sensor waveform from a Zirconium oxide type oxygen sensor. F: – This is the frequency value calculated by using the time interval between the two cursors. The time is inverted and displayed as a frequency (1/dT). In this case the Oxygen sensor switching frequency is ~1,2Hz.

A "slow" oxygen sensor will cause the air/fuel ratio to deviate more from the ideal range.

Oxygen sensor waveform from a Zirconium oxide type oxygen sensor. F: – Again, this is the frequency value calculated by using the time interval between the two cursors. The time is inverted and displayed as a frequency (1/dT). In this case the Oxygen sensor switching frequency is ~0,6Hz.

This type of malfunction can be caused by aging or poisoning of the sensing element. Causes of poisoning may include lead in the gasoline, excessive carbon from, for example, oil burning or anti freeze. Note, however, that the switching ratio is greatly affected by exhaust gas velocity. Notice that the switching ratio may be significantly slower at idle than at higher RPM. This depends to some degree on fuel system and intake manifold design, as well as distance from the exhaust valves to the sensor.
The transition time from lean to rich should not exceed 120ms and approximately 120ms rich to lean.

Oxygen sensor waveform from a Zirconium oxide type oxygen sensor. dT: – The time interval from low to high (from lean to rich) corresponds to ~78ms.

An increase in transition time can be caused by aging or poisoning of the sensing element. As earlier stated, causes of poisoning may include lead in the gasoline, excessive carbon from, for example, oil burning or anti freeze. Oxygen sensors will also age due to the harsh environment in which they work.
By analyzing the waveform from an oxygen sensor it is possible to diagnose problems in engine, engine management as well as the sensor itself.
The following waveform is from an engine with an operational oxygen sensor where there is a problem in the engine management system. This engine, when hot, will only idle for approximately 2 minutes before it loads up, due to an excessively rich fuel mixture. At the cursors a rapid opening and closing of the throttle is performed (snap throttle).

Waveform from a Zirconium oxide oxygen sensor mounted on an engine with an engine management problem.
A: – Is the voltage value at the cursor and is equal to ~800mV, indicating a rich mixture;
A-B: – Is the difference voltage between the two cursors at the moment of the snap throttle test and corresponds to a voltage value of ~700mV;
Snap throttle – A label marking the point in time where the throttle was sharply opened and closed.

In this waveform it is clear that at idle the voltage from the oxygen sensor was on the order of ~700mV with a deviation of ~ ±150mV. At snap throttle the voltage sharply decreased about ~700mV.
The sharp transition and voltage values indicate an oxygen sensor with acceptable performance. Since the oxygen sensor has acceptable performance but the engine will not idle for more than two minutes before loading up and the fuel mixture is consistently rich without the engine management being able to control it, there is an engine management problem. Also, during snap throttle, the fuel mix should very briefly be lean, then rich. In this case the mixture stays lean during the snap throttle period. The engine management problem could be caused by a bad sensor, wiring or connector. Additionally, the engine controller itself may be faulty. Remember that an engine management system able to control the air/fuel ratio should display an oscillatory oxygen sensor waveform. This is called closed loop and indicates that the engine controller is reacting to the changing air/fuel ratio.
The oxygen sensor can last anywhere from 20 000 – 200 000 KM, depending on the quality of the gasoline used, distance from the exhaust valves and the engine management system used. In general, oxygen sensors slowly deteriorate over their lifetime. The deterioration is usually first noticeable as a decrease in fuel mileage or a fluctuating idle. Often, a bad oxygen sensor will cease to function at idle where the exhaust gas temperature is lower, but function at higher speeds.

Oxygen sensor waveform from a Zirconium oxide type oxygen sensor. A: – The voltage value at the cursor is equal to about ~550mV.

The voltage value of the signal is almost stable, the value is close to 300-600mV.
If the temperature of the sensing element is increased, perhaps due to increased engine RPM, the sensor regains its ability to display a switching voltage.

Oxygen sensor waveform from a Zirconium oxide type oxygen sensor. A: – Voltage value at the cursor is equal to ~720mV; A-B: – voltage between the two cursors is equal to ~260mV.

The automotive technician can take advantage of the oxygen sensors ability to resume close to normal operation by raising the engine RPM and see if the oxygen sensor is capable of operating normally at cruise. In some cases the vehicle owner may elect to disregard the unstable idle and loss of fuel mileage associated with an oxygen sensor that cannot provide sufficient voltage for the engine management system to achieve closed loop. This should be discouraged due to the potential for damage to the catalytic converter and increased emissions. In some cases the engine management system will flag the sensor as faulty and set a trouble code for it.
In some cases the oxygen sensor can deliver a negative voltage, due to certain problems with the sensor itself.

Oxygen sensor waveform from a Zirconium oxide type oxygen sensor. A: – The voltage value at the cursor is ~45mV; A-B: – difference voltage between the two cursors is on the order of ~650mV; Snap throttle – A label marking a snap throttle occurrence.

With this type of malfunction the air/fuel ratio will usually become too rich, to the point of seeing carbon being emitted from the tailpipe and the spark plugs being covered in carbon as well.
Many engine management controllers are not designed to read a negative voltage, so the negative voltage will be interpreted as 0 V or a very lean mixture. The controller will then increase fuel delivery in an attempt at getting the proper voltage value from the sensor, causing the mixture to be very rich.
The inner chamber, which is the reference chamber, receives ambient oxygen through a reference channel or port. A negative voltage can occur if a lack of reference oxygen causes the oxygen content in the exhaust to be higher than in the reference chamber. Physical damage of the sensor or the sensor being coated with road salt, paint or undercoating can cause this problem to occur.
Many sensors receive their reference oxygen through the wiring harness going to the sensor. Something as simple as soldering these wires can cause this type of malfunction. It should be noted that many late model engine controllers are capable of reading negative voltages and may flag the sensor as bad, with an attendant trouble code. Decelerating in a fuel-cut mode can possibly cause a negative oxygen sensor voltage and may be considered normal.

Titanium oxide oxygen sensors.
The voltage value from these sensors can change from as low as a few mV, up to 4-5 V, depending on the sensing circuit in the engine controller. Titanium oxide sensors act as a variable resistor. They require a higher operating temperature to perform properly and are usually equipped with a heater. The heater is usually a PTC resistor, (Resistance increases with temperature) making them essentially self-regulating.

The waveform from a Siemens titanium oxide oxygen sensor. A: – The voltage value at the cursor is equal to ~4,5V; A-B: – The voltage difference between the two cursors is equal to ~4,4V.

These sensors react to the changing oxygen levels in the exhaust gas by changing their resistance. With a rich air/fuel ratio the sensors resistance is low and will, therefore, cause a larger voltage drop over the dropping resistor in the engine controller. An oscilloscope will show a low voltage with a rich mixture, the opposite of a zirconium oxide sensor. With a lean mixture the sensor has high resistance and will show a high voltage with a lean mixture. Some engine controllers are wired to measure the voltage drop across the sensor itself. These will show the conventional high voltage = rich mixture and low voltage = lean mixture as the zirconium sensors do.
These sensors, although accurate and fast, are seldom used. They tend to be very expensive.

Broadband Oxygen sensor.
The wide band or broad band type of oxygen sensor is essentially two oxygen sensors connected together. The first sensor is a "Nernst" cell and is used much like a conventional oxygen sensor. The second chamber is an "oxygen pump". An electrical current from the engine controller "pumps" oxygen ions through the membranes in this type of sensor. The size and direction of the current needed to pump oxygen ions in to or out of the sensing chamber to maintain Lambda=1 will vary according to the actual oxygen content of the vehicle exhaust.
Because ions are constantly being pumped through the sensor, it no longer acts as a simple switch. The direction and size of the current is an accurate measure of the actual air/fuel ratio. However, do not expect to see a normal oxygen sensor waveform when attaching the USB Autoscope II to the signal wire(s). At lambda=1 the output is 0 current and volts.