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How To Diagnose Slow or Sluggish Oxygen Sensors

An engine management system is always trying to find the perfect air/fuel ratio. But it is next to impossible to walk the line between too rich or too lean. With every revolution of the crankshaft, small changes in the air, fuel and operating conditions can cause changes to the oxygen content coming out of the exhaust port.

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An oxygen sensor measures the oxygen content in the exhaust. It does this by generating electricity from the oxygen differential between the exhaust and interior pump cell. It does not measure fuel, NOX or carbon monoxide. 

If you attached an oxygen sensor to a carbureted engine, the signal would not oscillate. It would be a flat line that goes up or down as fuel passes through the primaries and secondaries into the engine.

When oxygen sensors are tested, manufacturers will introduce small amounts of oil to measure sensitivity. 

On a fuel-injected engine, when you look at the oxygen sensor, the signal voltages produced do oscillate. The reason for the oscillations or changes in voltage is because the engine management changes the injectors’ pulse width and other parameters to achieve the perfect air-fuel ratio. It oscillates between rich and lean oxygen levels, but stays close enough so the engine runs smoothly.

As the oxygen sensor has evolved over the past 40 years, engineers have made the component sense a more extensive range of rich and lean conditions while also detecting small changes in oxygen content sooner. Some modern air/fuel sensors are so sensitive they can detect tiny leaks in the exhaust manifold. The sensor can read the missing gases as well as outside air getting pulled in as exhaust pulses pass the leak.


No matter the generation or materials at the heart of an oxygen sensor, they can be contaminated and no longer read the oxygen content in the exhaust stream. The enemies of platinum, zirconia or Titania are silicates, silicone and soot. These contaminants poison the component so that it can no longer measure the oxygen differential through the diffusion gap. Contaminants coat and bond with the surfaces and suffocate the sensing elements.

When an oxygen sensor is poisoned, it loses its ability to measure the oxygen content in the exhaust. The oscillations flatten out as the sensor becomes contaminated. The sensor signal eventually goes completely flat.

The engine management knows when not to trust a contaminated oxygen sensor. The engine knows how much air is coming into the engine using the MAS and MAP sensors. The module also knows how much fuel is being injected. If it does not see the switching from the sensor, it will set a code and start to control the fuel and spark to protect the catalytic converters.

Some engines might look at the downstream oxygen sensor to control the fuel trim. Downstream oxygen sensors (located in the exhaust stream after the catalyst) are used to monitor catalytic converter efficiency. In some engine management configurations, downstream oxygen sensor activity is used to adjust the air/fuel operation to maintain a favorable ratio to optimize catalyst efficiency. Other engine management systems might look at the opposite bank. 



If the oxygen sensor is contaminated, the most common codes are P0139 to P0153 for oxygen sensor circuit slow response. These codes are set when the engine management system sees a lower than expected signal voltage, or the sensor is not switching from rich to lean. Codes P0160 to P0166 for no O2 Sensor activity can indicate an issue with the circuit or the sensor is so contaminated the sensor no longer generates a voltage.

If the source of the contamination is far enough upstream, not only will the oxygen sensors be contaminated, the catalytic converters will be contaminated. If the contamination is not diagnosed before the sensor is replaced the sensor will set codes for catalyst efficiency and fail faster than the original.

Any time vehicle tailpipe emissions exceed 1-1/2 times federal limits, the EMS processor is programmed to record fault data. The Malfunction Indicator Lamp will illuminate after two consecutive faulted trips, and the oxygen sensor and its associated circuits are monitored for defects.

Sources of Contamination

If anything besides fuel or air reaches the combustion chamber, it has the potential to poison the oxygen sensor. The leading sources of contamination are engine oil and coolant. Both use additives containing compounds that can eventually damage an oxygen sensor. Oil formulators and automakers have been working at lowering the levels of zinc, phosphorus and silicates that can damage oxygen sensors and catalytic converters. New long-life coolants and GF-6 oils have replaced these harmful ingredients, but excessive leaks or blow by will limit the life of sensors and catalysts.


Other products like silicone gasket sealants and even anti-seize can damage an oxygen sensor. If the engine is not running an air filter, sand or silica can contaminate the oxygen sensor.


When a sensor fails, it must be replaced to restore normal engine operation. Diagnosis usually requires some additional testing once a fault code has been read. Accurate diagnosis is essential to prevent mistakes. Many sensors are replaced unnecessarily because they were misdiagnosed. For example, somebody read a code, assumed the sensor was bad and installed a new sensor. Sometimes it fixes the problem, and sometimes it doesn’t. Sensors are expensive, so they should not be replaced unless all other possibilities have been ruled out.

Most sensors do not have a recommended service or replacement interval, and most sensors are replaced after they have failed. Even so, a leading supplier of O2 sensors says replacing high mileage O2 sensors before they fail may improve fuel economy, reduce emissions and avoid annoying driveability issues. 


One of the most challenging problems to cure is an oxygen sensor code related to the heater circuit. If you decide to just put in a new sensor and clear the codes, there is a chance the code will come back because the malfunction in the circuit is not in the oxygen sensor.


The heater circuit on an oxygen sensor is not a coil of wire wrapped around the ceramic wafer or cone. Power is applied to the heater and warms the sensor’s elements that detect the difference of oxygen concentrations between the exhaust gases and the reference air in the pump cell. With a faster warmup, the engine can go into closed-loop operation sooner.

There are several types of circuit designs that provide power to the heater. On some older circuits, there will be a fuse and a relay. On some late-model sensors, there is only a module that pulls voltage to ground. But, the majority of systems provide power to the heater with pulse-width-modulated voltage.

Most heater systems test the circuit with a bias voltage. This checks the condition of the circuit before the pulse-width-modulated power is applied. If the engine control module detects an open, short or resistance that is higher or lower than expected, the system will go into a failsafe mode and won’t send power to the heater of the sensor.

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