Oxygen Sensors

WHAT IS OPEN LOOP VERSES CLOSED LOOP

OXYGEN SENSOR TYPES:

Zirconia sensors

The O2 sensor is mounted in the exhaust manifold, exposed to the exhaust gas stream. Information from the sensor is used by the ECM to fine tune fuel mixture. When the O2 sensor has reached an  operating temperature of over 300ºC, it acts as a voltage generator, producing a voltage of 0.1 to

1.0 volts, dependent on oxygen content in the exhaust gas.
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When the fuel system is operating correctly in closed loop mode, the sensor voltage output is changing several times per second, going above and below a rich/lean mid point range of 0.5 volts.  The ECM monitors this changing voltage, and determines the required fuel mixture correction.  An open circuit on the sensor or earth circuit, or defective, contaminated or cold sensor, could cause  voltage to stay within a 0.4 to 0.6 volt band for too long. A circuit voltage of over 0.5 volts for too  long indicates a rich exhaust. If a voltage of under 0.4 volts is monitored for too long, a lean exhaust  is indicated.
O2 sensors with 1, 2, 3 or 4 wires operate in the same way. The extra wires are required under certain operating conditions as a heater to maintain the sensors temperature above 400ºC. The  heater receives battery voltage.

One, Two & Three Wire Sensors

Single wire Zirconium sensors rely on the temperature of the  exhaust gas and system to heat them to their operating temperature. The single wire is the signal wire to the PCM. The signal is earthed through the sensor’s  body and the exhaust system. This can easily be affected by corrosion at any of the contact points  (thread, manifold to head, etc). If a large amount of corrosion occurs, the signal which is low voltage  and low amperage can be compromised.  Two wire sensors use an earth supplied by the ECU to clean up the signal quality.  Three wire sensors work the same as single wire sensors; however they have a heating element to  reduce the warm up time of the sensor. Usually the two white coloured wires leading into the sensor,  the heating element is supplied with battery voltage. The heating element has a Positive  Temperature Coefficient, as the heating element heats up the resistance of the element increases  slowing the current flow through the element or reducing the amperage required by the element. The  element’s resistance will not totally stop the current flow, thus there is always a quantity of current  although small flowing through the element.

Four Wire Sensors

Four wire sensors are now the norm on new vehicles. Essentially the same as a three-wire sensor, four-wire sensors have a designated earth for the sensor signal, which usually is connected straight  to the negative battery terminal. This relieves all concerns with corrosion affecting the signal. To  maintain the optimum operating temperature for the oxygen sensor the PCM applies a duty cycle to  the groundside of the heater circuit. This also allows the computer to ‘turn off’ the heater if needed.  Diagnosis of this type of sensor is best done using a scan tool to view the information the ECM is  receiving. The voltage from the sensor can still be measured though should remain steady at 450mV  while operating correctly.

When the fuel system is operating correctly in closed loop mode, the sensor voltage output is changing several times per second, going above and below a rich/lean mid point range of

0.5 volts. The ECM monitors this changing voltage, and determines the required fuel mixture correction.

An open circuit on the sensor or earth circuit, or defective, contaminated or cold sensor, could cause voltage to stay within a 0.4 to 0.6 volt band for too long. A circuit voltage of  over 0.5 volts for too long indicates a rich exhaust. If a voltage of under 0.4 volts is  monitored for too long, a lean exhaust is indicated.

O2 sensors with 1, 2, 3 or 4 wires operate in the same way. The extra wires are required under certain operating conditions as a heater to maintain the sensors temperature above  400ºC. The heater receives battery voltage and in most late model vehicles the heater is  negativley controlled by the ECM.

What are the common wiring colours of a Zirconium sensor?

1. Black – Signal +
2. Grey – Signal –
3. White – Heater +
4. White – Heater Note:

O2 sensors cannot be cleaned

Contamination of O2 sensors can be due to lead substitute additives, coolant discharged from head, engine oil, silicon sealants, prolonged over fuelling, leaking  injectors etc… Please also note that the manufactures will not give any warranty  on O2 sensors as they sample pollutants and contaminants.

“We suggest test don’t guess”

Titanium sensors

The one major difference to remember about Titanium sensors is that they require a voltage supply as they do not generate their own voltage.

The sensor will have 3 or 4 electrical connections because these sensors also require a high temperature to operate and therefore need a heater. The titanium electrode reacts to the oxygen  content in the exhaust system as does a Zirconium, though its resistance is the varying component  of this circuit. The voltage you can test for at these sensors is whatever voltage is supplied from the  ECM.

There are a couple of different ways manufacturers use these sensors.

In early use of the titanium sensors manufacturers would provide a voltage to the positive electrode and measure the voltage relative to earth. The negative electrode was connected to earth also. An  ECM can supply up to five volts for the sensor to modify as its resistance changes, the signal still  cycles, though it can cycle between a higher voltage than used to from a Zirconium type.

Common voltage supply is 1 volt or 5 volts supply.

The wiring colours for a Titanium sensor are:

Blue – Signal positive
White – Signal Negative
Black -Heater +
Black -Heater –

Typical Denso Sensor Pinout:

Mark Carlsson.
Dynojet / Dynatek Technical Manager.
SERCO PTY. LTD.

“Acknowledgements to Matt Hardy of Future Training for the images provided for me to use.”

Thanks Matt.