Easy Air-Fuel and Oxygen Sensor Diagnosis

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Easy air-fuel and oxygen sensor diagnosis

Truglia is the owner of Car Clinic, a state-of-the-art repair facility in Mahopac, N.Y. He is ASE A6 certified with a M.A. from Columbia University. In the automotive world he has been trained by Technicians Service Training and Automotive Technician Training Services. Car Clinic’s facility is fully equipped with state-of-the-art factory-level equipment and services American, European and Asian vehicles, including diesels and hybrids.

Vehicles diagnosed by Craig Truglia and Alex Portillo. Contributions made by G. Truglia, Kevin Quinlan and Adam Varney.

Some technicians who have worked in this business for years often still get confused by how to diagnose an air-fuel sensor or are not sure what to look at when diagnosing a rear oxygen sensor. In fact, when I started in this business (which is not that long ago) I was told that there is no way to diagnose an air-fuel sensor with a high level of certainty. Let me set the record straight: there are several ways that you can diagnose any air-fuel or oxygen sensor and be confident that you will be doing the right repair.

The basics

Why do we even have these sensors? O2 and air-fuel sensors are the vehicle’s personal emissions analyzer. These sensors measure how rich or lean the exhaust is.

Air-fuel and oxygen sensors work in tandem, before and after the catalytic converter. The PCM compares the readings in order to analyze catalytic efficiency, and whether the vehicle is running rich or lean.

We will get into diagnosing catalytic efficiency by looking at the rear oxygen sensor later, but first let’s make sure that we understand how oxygen and air fuel sensors regulate fuel control on a vehicle.

So, when the air fuel or oxygen sensor senses a rich fuel mixture in the exhaust, the PCM takes that information and then tries to do the opposite to make a fuel mixture that is perfect (called “Lambda”) by sending fuel trims in the opposite direction.

Because these sensors go bad at a relatively high frequency, it is important to understand how they should work and what approach we should take with their diagnosis.


Pattern failures

Before we get into theoretical details let’s make the following clear:

P0135 or P0141 heater circuit DTCs are almost always defective sensors that can be checked by using Ohms on your meter. “OL” indicates that an open heater circuit exists in the sensor and you should replace it.

A sensor obviously dead in the water not giving any feedback is most likely not a wiring problem. The easiest way to confirm this is to backprobe the sensor itself and look to see if it is showing voltage on your meter or labscope. Oxygen sensors generate their own voltage and if they show nothing they are obviously bad. Try taking one sensor out of a car and put it to the shop torch. You’ll see it makes its own voltage. (An air-fuel sensor also generates its own voltage, but it cannot be tested this way.)

Stick with the OE brand sensor. I have seen aftermarket sensors functionally be perfect with a good signal and working heater circuits, but they still set DTCs. Ignore the parts guy and just get the right sensor. Most Asian vehicles take Denso (sometimes NTK). Older American vehicles usually have Bosch but they have mostly moved over to Denso as well. European vehicles mostly use Bosch. Walker does not make their own sensors, but at a guesstimated 80% clip they rebox the OE sensor. If you are not sure what sensor the vehicle came with (and you cannot read it on the outside of the sensor) either buy it from the dealer first or remove it, bring it to the parts tore or dealer and match it up. Often you can buy the OE brand through the aftermarket, as long as you stick with the brand you took out of the vehicle.

Getting to know the oxygen sensor

The oxygen sensor measures the amount of oxygen in the exhaust that is used in the combustion process.

For pre-catalytic converter oxygen sensors used for fuel control:

Less oxygen than optimal in the exhaust results in a signal voltage over 450 mV. This reflects a RICH CONDITION. More oxygen in the exhaust than optimal results in a signal voltage under 450 mV. This reflects a LEAN CONDITION.

Good oxygen sensors have even waves in the 150 mV to 850 mV range while ascending or descending within a 100 mS or less when the system is in closed loop.

For post-catalytic converter oxygen sensors used for fuel control:

Post-cat oxygen sensors, when good, feature a steady voltage usually between 500 to 700 mV. If it zigzags, the catalytic converter is highly suspect.

On some vehicles the rear sensor does have some effect on fuel control. For our purposes, it’s just good to know that when testing the sensor, the voltage should go up when the fuel mixture is rich and should go down when it is lean. Sadly there is no way of generically knowing what is an optimal post-cat oxygen sensor voltage. It differs by the manufacturer.

Both front and rear oxygen sensors can be tested in the same way:

To make sure the sensor is reacting as it should to rich and lean conditions, simply cause a vacuum leak to make the system lean and use some propane to make the system run rich. You can do all of this by simply pulling out the brake booster hose. After you do this be sure to pump the brakes a couple times after you put everything back together. The sensor should react to rich and lean conditions instantly. If not, you might have a “lazy” sensor that needs to be replaced.

Mode 5 and Mode 6 tests

Even though Mode 5 is pretty much a thing of the past, both Mode 5 and 6 work the same. All they do is tell us if the PCM is happy with the feedback the oxygen sensors are giving it.

Mode 5 is not available on all vehicles besides some pre-CAN ones, but when it is you should view the data. The figures show how both Mode 5 and 6 provide voltage readings and switching results. The results can be helpful in making a decision concerning a P0420 DTC. If the front oxygen sensor voltage is not low or high enough, and is not switching in the correct time, you may not want to condemn that converter. When Mode 5 is not available Mode 6 should be used to view oxygen sensor test data.

Differences between the oxygen and air-fuel sensor

While both are used to measure catalytic efficiency and determine if a vehicle is running rich or lean, the way they work is fundamentally different. Air-fuel sensors reflect a lean condition when their voltage INCREASES and a rich condition when their voltage DECREASES.

Air-fuel sensors are only used for fuel control, so they are always a pre-catalytic converter sensor, not a post-catalytic converter sensor. The post-cat sensor is always a standard oxygen sensor. While a pre-cat oxygen sensor switches voltage from rich to lean, the air-fuel sensor stays at a steady voltage.



Getting to know the air-fuel sensor

The following are some important pointers:

* Don’t get confused by scan tool PIDS, since most scan tools will label A/F as an 02.

* Some generic/global scan tools do not display the true voltage. You will need a scan tool with accurate enhanced data. This is because OBD II standards require O2 sensor PID voltage to be displayed in a range between zero and 1 volt. Newer vehicles will have accurate air fuel sensor voltages.

* In generic OBD II what you often see is a percentage of true voltage. To display the actual PCM PID voltage, you’ll need a scan tool with the ability to read enhanced data or a scan tool with factory software. It’s pretty tough to accurately display voltage levels that start at 3.3 volts (Toyotas) using a 0 to 1 volt scale. The most common voltage reading on a Generic/Global scan tool is approximately 0.680 volts (again, Toyota).

You need to know specifications for air-fuel sensors

One of the toughest things about air fuel sensors is that no one tells you what a known good voltage is. Without knowing what your PID should be, it is very difficult to diagnose an air-fuel sensor.

The following are known good voltages for air-fuel sensors compiled over the last few years: 3.3 V (Toyota), 2.8 V (Honda), 1.9 V (Hyundai), 2.44 V (Subaru), 1.47 V (Nissan), 1.00 Lambda (all European manufacturers). Remember that a 1.00 Lamda is perfect while any movement above 1.00 (i.e. 1.01) is one perfect lean, and any movement below is rich by the same proportion. For example, a Lambda of 0.85 might set a system rich DTC with a LTFT of – 15%. Companies are not always forthcoming with this information, so you will have to compare voltages with known good vehicles in your shop.

Otherwise, you can connect your meter in series with the air-fuel sensor in amps mode. A perfect reading is zero amps. Each milliamp above zero is a percentage point lean and each milliamp below zero is a percentage point rich. This works fundamentally the same as emissions analysis.

Diagnosing air-fuel sensors

The air-fuel sensor can be tested just like an oxygen sensor by forcing a lean and rich condition, making sure the sensor quickly and accurately responds. If you have a voltage specification, you can make sure the sensor is accurately responding to rich and lean conditions and compare what you see to what you know to be good.

The air-fuel sensor is going to have little humps when graphed. The post-cat oxygen sensor paired with it should not oscillate, but instead stay pretty steady somewhere between 500 to 700 mV.

Fundamentally, air-fuel sensors work just like regular oxygen sensors, but in a mirror fashion. When the condition is rich they decrease in voltage. To the contrary, when the condition is lean their voltage spikes. This is opposite of our normal inclination to view high voltages as a rich indicator, and low ones as a lean indicator, so be careful.

As we can see, as throttle position and engine speed go up and the mixture gets richer, the voltage declines. Voltage goes up when engine speed and throttle position decline, as the mixture is leaned out to bring the car back to a proper air-fuel mixture.


Oxygen/air-fuel sensors and catalytic converters

Oxygen and air-fuel sensors should act in a predictable manner, since that’s their job. They are placed before and after the catalytic converter (only oxygen sensors) so they can check if the converter is cleaning up emissions.

If the cat is working right, it will clean up emissions, and the sensors will report this information back to the PCM.

Before the catalytic converter, the oxygen sensor will zigzag up and down. To the contrary, the air-fuel sensor will be at a stable voltage. The post-cat oxygen sensor will be a straight line if the catalytic converter is good in most cases.

If the catalytic converter is bad, the post-cat oxygen sensor will mirror the post-cat oxygen sensor. Sometimes the post-cat oxygen sensor will have a gap of time between the pre-cat sensor switching voltage and itself. This is often normal during a sudden fuel event that the catalytic converter, even when good, cannot instantly clean up.

Real-world air-fuel sensor diagnosis: 2002 Subaru Forester P0130 and P0171

One of our best customers brought their vehicle in because it had the check engine light on. The vehicle otherwise ran fine. So, she brought the vehicle and the light happened to be off at that moment. So, we did the oil change and sent the vehicle on its way. A few minutes after she left the check engine light popped back on. This is when the fun began.

The first thing we did was scan the codes.

After this we checked for TSBs and finding none, we looked for hits on Identifix. Apparently a lot of air-fuel sensors go bad, but the test recommended by Identifix perplexed us. It said to replace the sensor if the post-cat oxygen sensor was rich while short term fuel trim was lean.

Graphing the data showed some interesting results.

Obviously the STFT was totally off and was indicative of what would have been an oxygen sensor shifted lean or a major vacuum leak. The method that Identifix recommended was to look at the rear oxygen sensor data to see if it was “rich,” which would obviously indicate that the air-fuel sensor was stuck lean and thereby commanding fuel until the system was in reality rich though theoretically running lean. This appeared to be what was happening.

The rear oxygen sensor was at 800 mV, which is on the high side... I guess. However, that’s just not enough of a smoking gun for us.

So, we needed to find out if the air-fuel sensor was meeting specification. The Autoland Scientech Vedis II had a PID that gave the air-fuel ratio sensor as Lambda. Pardon the bad picture, but these screen captures are taken during real shop conditions. As you can see, the Lambda was elevated into lean territory, here captured at 1.21.

We added propane and the sensor would not budge. It was pinned lean.

A few minutes after we were done with the test the sensor started working well again and the Lambda fell to 1.00. STFT was normal. As far as we were concerned, we caught an intermittently bad air-fuel sensor in the act. However, we wanted to get the voltage specification for this vehicle when it was good, because manufacturers tend to use the same voltage for all the vehicles they have.

Testing this sensor required no fancy back probing or looking up anything on a wiring diagram. The sensor had a cover over an area with a positive and negative sign, designed to place to meter leads (Figure 1). On our meter we read 2.44 V. We simply replaced the sensor, checked Lambda, and were happy with what we found. The car was sent on its way and has not been back since.

Wrapping it all up

Oxygen sensors nor air-fuel ratio sensors are very complicated. They simply tell the PCM if the vehicle is running rich or lean. Where good technicians get confused is that for years they have been working on oxygen sensors and they do not understand that the air-fuel works in a fundamentally different way.

However, air-fuel sensors have been around for over 10 years on many vehicles. We need to know how they work as second nature. With the right specifications and testing methods covered here, there’s no reason why you cannot diagnose these sensors easily and quickly.



NASCAR is now using injection

As NASCAR replaces carburetion with fuel injection in Sprint Cup racing in 2012, Bosch is the exclusive oxygen sensor supplier for the new engines. An official NASCAR Performance Partner, Bosch supplies two special-for-NASCAR wide-band oxygen sensors for each vehicle. These sophisticated sensors will provide essential data to control the race cars’ fuel injection engine management systems.

“Two Bosch wide-band oxygen sensors, one on each engine bank, relay variable information on engine performance, virtually continuously, to the vehicle’s fuel management system, which controls the fuel injectors and determines how the vehicle reacts to race conditions. This change to fuel injection will give NASCAR drivers enhanced control over their vehicle’s performance as well as fuel consumption. Oxygen sensors are vital to obtaining maximum performance on each track,” said Wolfgang Hustedt, Bosch motorsports manager, North America.

How do oxygen sensors work to provide this highly valuable function?

It all began in 1899 when Professor Walter Nernst, in Leipzig, Germany, developed the theory of a “concentration cell” which, much like a battery, uses a gas-tight ceramic electrolyte, which becomes electrically conductive at temperatures above 625 to 650° F. This “Nernst cell” transfers oxygen ions from “reference air” inside the cell to the outside environment (the exhaust stream), or from the outside environment to the reference air in the cell. This flow of ions generates measurable voltage, reflecting the difference in the oxygen content between the gas outside the sensor and the reference air inside the sensor.

The oxygen content indicates whether the exhaust gases are “rich” or “lean,” and Bosch engineers used the basic Nernst theories and experiments to create the very first automotive oxygen sensor. Following extensive experimentation, testing and engineering development, Bosch’s pioneering automotive oxygen sensor was installed for the first time in a 1976 Volvo.

The goal of the oxygen sensor is to help the engine’s fuel management system approach or maintain the ideal 14.7:1 stoichiometric air to fuel ratio. In nearly all oxygen sensors, a lean mixture (greater than 14.7:1) causes the oxygen sensor output voltage to drop, while a rich mixture (less than 14.7:1) causes the sensor output to go up. If the mixture is perfectly balanced at stoichiometric, the sensor sends a minimal signal (about 0.45 volts) which tells the vehicle computer the air/fuel mixture is correct.

The reaction speed of oxygen sensors to changes in the exhaust oxygen level is determined by the sensor itself and by the type of fuel delivery system the engine is using. Oxygen sensors used with older feedback carburetors switch once every second at 2,500 rpm. Sensors installed with throttle body fuel injection systems switch two or three times per second at 2,500 rpm, while newer sensors installed with multipoint fuel injection systems can switch five to seven times per second at 2,500 rpm.

Wide-band sensors provide variable readings

The highly sophisticated Bosch heated wide-band oxygen sensor, as used by NASCAR, utilizes an internal layered ceramic strip and adds a whole new concept — a “pumping cell.” This pumping cell allows the wide-band sensor to accurately measure the air/fuel ratio and produce a variable signal, virtually continuously, that reports readings all the way from very rich to very lean, and anywhere in between, rather than simply either “rich” or “lean” as with other sensors.


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