Understanding O2 Sensors

Having had a few questions coming from customers about O2 sensors, I decided to pen a few words. This sensor is always claimed to be the cause of your check engine light indicator coming on. So you spend 200 bucks and be on your merry way thinking that the problem will disappear. More often then not, this doesn't happen. 

And then there are those that takes out a nifty little obd2 tool to read the ECU fault codes from your car computer. Many times they would diagnose your O2 sensor as faulty even after retrieving the codes unrelated to the O2 sensor because either they don't understand what the codes are or they hope that somehow magically replacing the O2 sensor would solve other problems.

First of all, we do not know if your O2 sensor is actually faulty because the fault codes only tells you that the O2 sensor is detecting rich or lean for a predefined period of time. So the issue may be caused by something else.

In order to determine if the O2 sensor is in good working condition, you will have to tap the signal off your O2 sensor with a multimeter. During normal car operation, the voltage readings are supposed to always be varying between high and low - between lean and rich. A sensor that stays at a certain reading especially during driving conditions is likely to be broken.

Tapping the signals are not considered an easy thing to do especially while driving and this is where the OBD2 Bluetooth adapter can help you in logging the readings for data collection (Check my thread on the OBD2 Adaptor on the left pane). Only after this can you determine if your O2 sensor is indeed faulty. Sometimes a simple cleaning to the O2 sensor can be enough to dislodge carbon and make it operational again.

You can read on further below for a more in depth reading of the O2 sensor operation.

All About Lambda Sensors

The exhaust gas oxygen sensor (EGO or O2), or lambda sensor, is the key sensor in the engine fuel control feedback loop. The computer uses the O2 sensor’s input to balance the fuel mixture, leaning the mixture when the sensor reads rich and enriching the mixture when the sensor reads lean.
Lambda sensors produce a voltage signal that recognises the amount of unburnt oxygen in the exhaust. An oxygen sensor is essentially a battery that generates its own voltage. When hot (at least 250 °C), the zirconium dioxide element in the sensor’s tip produces a voltage that varies according to the amount of oxygen in the exhaust compared to the ambient oxygen level in the outside air. The greater the difference, the higher the sensor’s output voltage.

Sensor output ranges from 0.2 volts (lean) to 0.8 volts (rich). A perfectly balanced or "stoichiometric" fuel mixture of 14.7 parts of air to 1 part of fuel gives an average reading of around 0.45 volts.
The lambda sensor’s output voltage doesn't remain constant, however. It flip-flops back and forth from rich to lean. Every time the voltage reverses itself and goes from high to low or vice versa, it’s called a “cross count”. A good O2 sensor on a injection system should fluctuate from rich to lean about 1 per second. If the number of cross counts is lower than this, it tells you the O2 sensor is getting sluggish and needs to be replaced.

Most lambda sensors will cycle from rich to lean in about 50 to 100 milliseconds, and from lean to rich in 75 to 150 milliseconds. This is referred to as the “transition time”. If the O2 sensor is taking significantly longer to reverse readings, this too is an indication that it is getting sluggish and may need to be replaced.

Observing the sensor’s waveform on a scope is a good way to see whether or not it is slowing down with age. If the sensor becomes sluggish, it can create hesitation problems during sudden acceleration.

Heated Oxygen Sensors

To reduce the warm-up time of the lambda sensor, an internal heating element may be used. Heated O2 sensors can reach an operating temperature of as high as 500 degrees C in as little as eight seconds! Shorter warm-up time means the system can go into closed loop fuel control sooner, which reduces emissions and improves fuel economy. Heating the sensor also means it can be located further downstream from the exhaust manifold.

 

Titania Oxygen Sensors

Some vehicles have a slightly different type of sensor that has a titania element rather than zirconia. Titania O2 sensors are fitted to some Vauxhalls.

 

The operating principle of a titania lambda sensor is entirely different from that of a zirconia lambda sensor. A titania lambda sensor works like a coolant sensor. It changes resistance as the air/fuel ratio goes from rich to lean; but instead of a gradual change, it switches very quickly from low resistance (less than 1000 ohms) when the mixture is rich to high resistance (over 20,000 ohms) when the mixture is lean.

 

The input end of the titania sensor is fed from a fixed 1-volt supply, and the output end is pulled towards 0 volts by a fixed resistor. As the resistance of the titania sensor varies, so does the voltage at its output. When the fuel mixture is rich, the resistance of the sensor is low and so its output voltage is high. When the fuel mixture is lean, resistance shoots up and the voltage signal drops.

Taken from http://www.picoauto.com/applications/lambda-sensor.html