Testing Oxygen Sensors

Types of Oxygen Sensors

There are a few common types of oxygen sensors found on vehicles, which have between one and five wires connecting them to the rest of the vehicle. You must identify what type of oxygen sensor you are working with before trying to do any test:

• Zirconia sensors, also known as ‘narrowband oxygen sensors’, these are the most common type. Zirconia sensors have two electrodes, which output 200mV (0.2V) in a ‘lean’ condition, and 800mV (0.8V) in a rich condition. In a normally-functioning engine, zirconia sensors usually output 450mV (0.45V).
• Wideband zirconia sensors, often simply called ‘wideband sensors’ are also fairly common. Wideband sensors have four electronic connections, one pair of which is their output signal.
• Titania sensors, which are a type of narrowband sensors that is uncommon but not rare. There are two kinds of Titania sensor, one of which operates at a full-scale range of 5 Volts, and the other of which operates at 1 Volt.

Oxygen Sensor Locations

Oxygen sensors are usually located in one of two spots (along the engine exhaust), and it is important to know which you are dealing with. The positions are:

• Between the exhaust manifold and the catalytic converter. These are known as ‘pre-catalytic converter’ or ‘pre-cat’ sensors
• Between the catalytic converter and the exhaust outlet (tailpipe). These are known as ‘post-catalytic converter’ or ‘post-cat’ oxygen sensors.

Pre-catalytic converter oxygen sensors usually output a signal which varies between ‘lean’ and ‘rich’ (or high and low). The Post-catalytic converter oxygen sensors generally have a smooth output, as the catalytic converter mixes the remaining uncombusted exhaust, and reacts the oxygen with the fuel.

Oxygen Sensor Tests

‘Testing an oxygen sensor’ can mean many different things. The most common tests are:

• Oxygen sensor heater tests. This is usually a test of the heater element’s resistance or power consumption, done with a multimeter or clamp-meter.
• Oxygen sensor average output level tests. This is a test of the average output of a sensor, done with a multimeter.
• Oxygen sensor crossing count tests. This is a test of the oxygen sensor’s behaviour on a running engine, done with an oscilloscope or ST05 Oxygen Sensor Tester/Simulator.
• Oxygen sensor response tests. These vary widely, but are usually done with a propane torch (or other heat source), and some measurement device (like a multimeter or ST05 Oxygen Sensor Tester/Simulator).
• California Air Resources Board oxygen sensor response test. This is a special test (described below), which was never widely adopted.

California Oxygen Sensor Test

In the 1990s, the California Air Resources Board introduced a standard for testing automotive oxygen sensors. To pass this test, the oxygen sensor must transition from ‘low’ to ‘high’ in less than 100ms, when the engine is warm and running at 1800 RPM. For various reasons, the test was never widely adopted in the automotive industry, so most oxygen sensors do not pass the test, even when brand-new and functioning properly. You should not rely on the test unless the oxygen sensor manufacturer explicitly states that their device is compliant with California’s test.

How to test an Oxygen Sensor with a Multimeter

The easiest test to do on an oxygen sensor with a (digital) multimeter is to check whether the heating element is broken (assuming the sensor in question is self-heated). You can check the oxygen sensor’s heating element by

1. Switching the multimeter on, to ‘resistance’ mode.
2. Connect the test leads to the heater power and ground connector pins or wires.
3. Read the multimeter measurement, most of these heaters have an internal resistance of about 10Ω to 20Ω (when cold).

The next test you can run on an self-heated oxygen sensor is to check whether its heating element is being powered. To do this test:

1. Make sure the engine exhaust system is cold. Some heaters will not turn on if the engine exhaust pipes are hot.
2. Switch the multimeter on, to ‘DC voltage’ mode.
3. Connect the multimeter to the heater’s power wires or contacts. Back-probes are the best tool for this. If you do not have access to back-probes, it may be easiest to connect the multimeter to the power lines by disconnecting the oxygen sensor from its harness, and connecting the multimeter to the connector. You should read the engine service manual to learn what you can and cannot do here.
4. Turn on the engine.
5. Observe the voltage reading on the multimeter, it should be between 12V and 14V.

If you are working on a wideband zirconia sensor, you also can try to check its average output voltage, which should usually be around 450mV and stable when the engine is running and warm. Narrowband sensors (zirconia and titania), especially pre-catalytic converter, are tough to test with a multimeter. Multimeters do not respond fast enough to catch a narrowband sensor’s rapidly changing output.

How to test an Oxygen Sensor with a Clamp-meter

Clamp-meters make testing the oxygen sensor’s self-heating much faster and easier. All you have to do is:

1. Make sure the engine exhaust system is cold.
2. Switch the clamp-meter on, to ‘DC current/DC amperage’ mode.
3. Put the clamp around either of the oxygen sensor heater power wires (but not both). Be careful not to rest your hand or the tool on the engine or exhaust
4. Turn the engine on.
5. Observe the reading, which should be between 0.25A and 1.5A.

Some of the advantages of using the clamp-meter (when compared to the conventional multimeter) are that it is quicker, more informative, and less intrusive, as it does not interfere with the engine’s normal operation.

How to test an Oxygen Sensor with an Oscilloscope

Oscilloscopes are very useful tools, and much more informative than multimeters, but they can also be challenging to use with oxygen sensors. It is usually best to use either a battery-powered oscilloscope or one with isolated inputs, as vehicles may not share a ground with the mains power in a garage or shop. If a vehicle ‘floating’ above or below the voltage of the oscilloscope’s power supply can discharge a significant current, at thousands of volts, damaging vehicle circuits or an oscilloscope. The second challenge when using an oscilloscope to test oxygen sensors is actually connecting the oscilloscope to the oxygen sensor’s circuit(s), which is best accomplished with back-probes. To use an oscilloscope on an oxygen sensor, you should:

1. Ensure that your oscilloscope inputs are properly isolated from garage or shop mains power.
2. Make sure the engine is cold.
3. Connect the oscilloscope probes to the oxygen sensor cell lines (making sure to use the oscilloscope’s reference/ground clip). Make sure the wires will not interfere with moving parts on the engine.
4.  Start the engine
5. Observe the oxygen sensor outputs while the engine is running, and over time. Oxygen sensor outputs should be low while the engine warms up, then rise to have an average value corresponding to a ‘balanced’ mixture. Pre-catalytic converter sensor outputs should usually oscillate quickly between ‘rich’ and ‘lean’. Post-catalytic converter outputs should be much more stable, around the ‘balanced’ level. The number of times that the signal crosses its average is an important parameter, and each system (ECM/PCM, engine, and oxygen sensor) has a characteristic number of crossings per second.
6. Turn off the engine.
7. Wait until the engine is cold.
8. Remove the oscilloscope probes.

Testing with an ST05 Oxygen Sensor Tester

Our very own ST05 Oxygen Sensor Tester/Simulator is probably the best, and easiest-to-use tool for checking oxygen sensors. The ST05 will not damage oxygen sensors, and comes with special clips, which can either be clipped onto exposed metal, or used to pierce the signal wires (where that is acceptable).

1. Make sure the engine is cold.
2. Connect the ST05 test leads to the oxygen sensor’s outputs. The ST05 will let you know (via the alphanumeric display on its right side) if it detects a misconnection, such as no connections, a connection to the heater wires, or reversed polarity.
3. Turn on the engine.
4. Observe the ST05’s displays, as the engine warms, and over time. The oxygen sensor’s output (displayed on the left bar on the ST05) should generally start off low, and rise as it warms up. When the engine is warm, you can see the ‘crossing count’ on the right display. The number of times that the signal crosses its average is an important parameter, and each system (ECM/PCM, engine, and oxygen sensor) has a characteristic number of crossings per second. Pre-catalytic converter sensor outputs should usually oscillate quickly between ‘rich’ and ‘lean’. Post-catalytic converter outputs should be much more stable, around the ‘balanced’ level.
5. Turn off the engine.
6. Wait until the engine is cold.
7. Remove the ST05 test leads.

In summary, the ST05 can provide you about as much information about an oxygen sensor on a running engine as an oscilloscope does, while being less expensive, and much easier to use. The ST05 can also drive the oxygen sensor’s output (the ECM/PCM input) ‘lean’ or ‘rich’ (low or high), which is often useful for testing, but that is beyond the scope of this post.

That’s All!

If you’re interested in more information about our oxygen sensor tester, you can find it at the ST05 Oxygen Sensor Tester/Simulator product page. If you have any ideas for topics we should cover in future blog posts, please send us an e-mail.