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Oxygen sensors, also known as O2 sensors, Lambda sensors, Hego sensors, A/F sensors and AFR sensors, have been 

fitted to Australian vehicles since the mid 1980s (eg. Holden VL Commodore RB30), but have been used in overseas 

vehicles earlier than this. 


The job of the oxygen sensor is to relay oxygen levels passing through the exhaust to the ECU to maintain the correct 

air / fuel mixture of 14.7:1 (stoichiometric ratio, lambda 1) The ECU then uses this information and to adjust the 

amount of fuel injected into the air intake stream. This process is known as closed loop operation. 

The ECU in open or closed loop condition is dependent on engine temperature, engine load, engine speed, throttle 

position and oxygen sensor operating temperature (heating circuits). An ECU uses the closed loop operation during 

part throttle/cruise conditions to maintain the correct air/fuel ratio. Open loop operation is when the ECU uses a 

predetermined fuel map, therefore the ECU ignores the oxygen sensor output. Open loop conditions are usually idle, 

acceleration and wide open throttle. Later models tend to operate in closed loop at idle. 

There are different types of oxygen sensors, but two of the most common types are: 

- Narrow band oxygen sensor (up to 4 wires), the older style, simply called oxygen sensor. The output range is 

0 - 1 volt. Typical operation should show the voltage cycle between 0.1 volts (rich) and 0.9 volts (lean) and 

should fluctuate every second (1Hz). When fitted post-catalytic converter (rear sensor also known as 

diagnosis sensor) this sensor should have a flat line output of approximately 0.5 volts and may been seen as 

a wave form using the latest scanners. 

- Wide band oxygen sensor (commonly 5 wires), the newer style, called air/fuel ratio (A/F, AFR) sensor. This 

sensor does not cycle like a narrow band sensor. The output of 0 - 5 volts is processed at the ECU and is 

displayed as an air/fuel ratio (AFR) through live data on a scanner. 


Diagnosing faulty oxygen sensors requires the correct test equipment, a scanner that has the ability to show wave 

forms, a multimeter with bar graph and an oscilloscope. Testing should be performed at idle, steady 2000-3000 RPM 

and snapped throttle to observe the sensor behaviour. Common symptoms of faulty oxygen sensors may include 

diagnostic trouble codes, excessively lean or rich mixtures, surge while driving, lack of power, rough idle and 

excessive fuel consumption. 


Oxygen sensor heater circuits (like a glow plug built into the oxygen sensor) can draw in excess of 8 amps. Some 

heater circuits are designed to be ‘on’ when the engine is running, and others may be controlled through the ECU by 

digital switching. Many car manufacturers specify the routine replacement of oxygen sensors. eg Mercedes Benz ML320 (W163) 

recommends replacement at 160000km. 


Factors contributing to the premature failure of oxygen sensors may include driver habits, contamination from 

engine coolant, excessive oil consumption, silicon sealants, additives and incorrect or poor fuel. 

Tech tips for quick diagnosis of faulty oxygen sensors:


- If symptoms include lean or rich conditions, surging, lack of power, rough idle and excessive fuel 

consumption. Disconnect the front oxygen sensor and test drive. If the symptoms disappear when the sensor 

is disconnected, the oxygen sensor is faulty. The sensor output should be tested to confirm diagnosis.   

- If you suspect a faulty oxygen sensor on V type engine or an engine with 4 sensors (B1 S1, B1 S2, B2 S1, B2 

S2), swap the sensor (same part number) over to see if fault follows by observing the DTCs. 

- Many vehicles especially Subaru and Toyota vehicles are logging heater circuit related fault codes. The ECU 

monitoring sensor current flow has recognised a problem with the heater circuit. The remedy generally is to 

replace the sensor (part numbers may be required when ordering). Confirm diagnosis by checking the 

continuity of the sensor wiring. 

- If the customer complaint is excessive fuel consumption and all diagnostic tests show no faults. The oxygen 

sensor may be out of calibration. Replacement of the oxygen sensor is recommended. Note: Explain to the 

customer briefly what the sensor does and why it should be replaced. 

- To test the response of an oxygen sensor, induce a lean or rich mixture (create vacuum leak or add 

hydrocarbons to engine inlet) and monitor live data and fuel trim readings through a scanner. Readings 

should correspond with changing conditions. 

- P0170-P0175 (system too lean/fuel trim reached limits). This code means the ECU can no longer adjust the 

fuel mixture in closed loop operation. Check air mass meter, map sensor, vacuum leaks, fuel pressure/flow 

and fuel injectors before replacing the oxygen sensor(s). 

- P0420 (catalytic converter efficiency below threshold). This code P0420 has logged because the ECU has 

noticed the rear sensor is behaving like the front sensor. The front and rear oxygen sensor must be checked 

for correct operation by waveform using a scanner or a 2 channel oscilloscope. Ensure that there is no air 

being drawn into the exhaust system and the engine has no other faults. Incorrect outputs from either or 

both front and rear oxygen sensors or a failed catalytic converter may cause this problem. 

- Final testing of the oxygen sensor(s) with an oscilloscope must be performed to confirm diagnosis. 

- The simplest way to determine which sensor position you are looking for is to disconnect an oxygen sensor 

to get the location (B1 S1, B1 S2, B2 S1, B2 S2 etc.) through scanner fault codes.


 For parts and technical assistance contact your local CoolDrive EFI Division Branch 








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Neither is any liability assumed by CoolDrive for damage or loss resulting from reliance on the information or process disclosed.