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Advanced O2 sensor diagnostics: Tracing sensor wiring and checking for ‘lazy’ sensors

Figure 1: Depicted here is a graphic representation of how the PCM reacts when the front oxygen sensors detect an air-fuel mixture above or below Lambda.
<p>Figure 1: Depicted here is a graphic representation of how the PCM reacts when the front oxygen sensors detect an air-fuel mixture above or below Lambda.</p>

Over the years we covered a lot of the basics concerning oxygen and air-fuel sensor diagnostics. This includes their basic operation, the differences between oxygen and air-fuel sensors, air-fuel ratio sensor voltage specifications, the differences between zirconium and titanium dioxide oxygen sensors, and using emissions analyzers to verify Lambda.

If we can understand all of the preceding, there is not much more that we need to know in order to test these sensors. However, we have not yet covered tracing O2 sensor wiring and checking for a “lazy” sensor. After reviewing the basics, we will move onto troubleshooting these more advanced oxygen sensor issues.

Understanding fuel control

Oxygen and air-fuel sensors are the vehicle’s personal emissions analyzer. These sensors measure how rich or lean the exhaust is.

The “front” sensors, located in front of the catalytic converter, are the ones that are used to determine fuel control. On a scan tool, the B1S1 and B2S1 are the front sensors, as the “S” means “side,” and “1” means that it is in front of “2,” which would be the rear sensor. The B stands for “bank,” and 1 and 2 indicates which bank of the motor is being referenced. Being that motors are in all different positions in different vehicles, the technician needs to look up the firing order on an information system in order to know which bank is 1 or 2. The front sensor is always on the same side as cylinder number one in the firing order.

Why are vehicles designed this way? Obviously, if the exhaust goes through the converter, the emissions are cleaned up and this affects the signal. For this reason, the front sensors are located in front and are designed to be sensitive so that they can detect fluctuations in the air-fuel mixture.

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. The same is true when the oxygen sensor senses a lean fuel mixture in the exhaust. When this occurs, the PCM takes this information and adds fuel trim to enrich the air-fuel mixture in order to achieve Lambda again. Figure 1 shows us how this works and provides a graphic representation of how the PCM reacts when the front oxygen sensors detect an air-fuel mixture above or below Lambda.

When we look at a zirconium or titanium dioxide oxygen sensor on a scan tool, the technician can see these adjustments in real time. The waveform tends to oscillate above and below 450 mV. When the sensor trends above 450 mV more often than not this indicates a rich condition and below this voltage indicates a lean condition. A titanium dioxide sensor will read the same on generic OBD II, but on Chrysler products this reading in OEM-enhanced datastream is between 2.6 to 3.4 V, 3.0 V being perfect Lambda.

Air fuel sensors work the opposite. They reflect a lean condition when their voltage increases and a rich condition when their voltage decreases. This is identical to how Lambda works on an emissions analyzer. Above 1.0 is lean while below is rich.

Diagnosing ‘lazy’ oxygen sensors

Oftentimes, when the technician has a performance-related DTC, the oxygen sensor looks like it is operating normally. Back before scan tools offered graphing capabilities, it was time to whip out the labscope.

Good oxygen sensors tend to 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. An oxygen sensor usually switches between a high to a low voltage and back to a high one again in less than a second. Of course, this is only a rule of thumb and not true of every vehicle. Some may switch much quicker, while others would switch slower. Refer to Figure 2: This oxygen sensor, even though it is bad (it is shifted rich), switches within the correct amount of time. The labscope being used here is the EScope from Automotive Test Solutions.

Figure 2: This oxygen sensor, even though it is bad (it is shifted rich), switches within the correct amount of time. The labscope being used here is the EScope from Automotive Test Solutions.
<p>Figure 2: This oxygen sensor, even though it is bad (it is shifted rich), switches within the correct amount of time. The labscope being used here is the EScope from Automotive Test Solutions.</p>

Is there an easy way to find out on a scan tool? Perhaps. Many scan tools do not have the update rate that a labscope has, so even if its graphing function shows a time division, it might not be an accurate reading.

Nonetheless, on vehicles with motors that have two banks, there is a way to test the oxygen sensor without checking a known good sensor on an identical vehicle. Simply graph both S1 and S2 oxygen sensors at the same time. Their voltage oscillation should be identical. If one goes up and down quicker than the other, and the slower one is the one that happens to have the DTC, the slower one is “lazy.” Now it’s time to get a new one that will work quicker!

See Figure 3: The top two columns on this screen shot show the B1S1 and B2S1 oxygen sensors. As one can see, both go up and down around the same time. One is not faster than the other. By using a scan tool that graphs multiple PIDs at the same time, such as the EScan as pictured here, picking out a lazy oxygen sensor is a snap.

Figure 3: Shown here is an example of reading multiple PIDs. The top two columns on this screen shot show the B1S1 and B2S1 oxygen sensors. As one can see, both go up and down around the same time. One is not faster than the other. By using a scan tool that graphs multiple PIDs at the same time, such as the EScan, picking out a lazy oxygen sensor is a snap.
<p>Figure 3: Shown here is an example of reading multiple PIDs. The top two columns on this screen shot show the B1S1 and B2S1 oxygen sensors. As one can see, both go up and down around the same time. One is not faster than the other. By using a scan tool that graphs multiple PIDs at the same time, such as the EScan, picking out a lazy oxygen sensor is a snap.</p>

When in doubt, look at Mode 6. If the oxygen sensor PID is showing movement and the DTC is not for a heater circuit, then the technician knows that the PCM is receiving the data. By default, if the technician looks at Mode 6 when there is a DTC, the sensor is probably going to have failed the test.

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