Tech Stuff

2014 oxygen and air/fuel sensor update

 

FIGURE 1: This 2008 Jeep Grand Cherokee 5.7L has oxygen sensors that read between 2.6 and 3.4V when we use enhanced OBD II on an OTC Genisys.
<p>FIGURE 1: This 2008 Jeep Grand Cherokee 5.7L has oxygen sensors that read between 2.6 and 3.4V when we use enhanced OBD II on an OTC Genisys.</p>

Truglia is the owner of Car Clinic, a state-of-the-art repair facility in Mahopac, N.Y. He is ASE 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 factory-level equipment and services American, European and Asian vehicles, including diesels and hybrids. (All vehicles were diagnosed by G. “Jerry” Truglia, Alex Portillo and Craig Truglia.)

Back in the day when electronic fuel injection and computers in vehicles were first coming out, oxygen sensors were new. Technicians freaked out wondering how they worked and had to figure out new ways to diagnose vehicles. Eventually, they got used to the pattern failures (i.e. oxygen sensors stuck lean, bad heater circuits, etc.) and understood how they worked: the richer the fuel mixture, the higher the voltage and the leaner the fuel mixture, the lower the voltage.

Then, air/fuel sensors came out and also confused technicians worldwide. The sensors worked in the reverse fashion, and instead of switching between high and low voltages, they used a steady, unspecified voltage. As recently as 2010, a young, ASE L1 with 12 years in the field told me, “There is no way to properly diagnose air/fuel sensors.”

However, as more time passes, technicians are getting increasingly familiar with diagnosing these sensors. Nonetheless, there are still some changes afoot that are worth tracking.

‘New-school’ oxygen sensors

Many service techs thought that the days of having to learn anything about oxygen sensors were over. Think again. Chrysler vehicles still exclusively use oxygen sensors, as opposed to air/fuel ratio sensors, to give the powertrain control module (PCM) feedback in order to calculate whether combustion is stoichiometric, or in plain English, if the exhaust is rich or lean.

However, intent on not making the lives of technicians easy, vehicle manufacturers are beginning to employ oxygen sensors that read between voltages higher than 1V. In fact, the front and rear oxygen sensors on Chryslers for the last few years, including 2014, are 5V sensors. On an example using a 2008 Jeep Grand Cherokee 5.7L (see Figure 1), the oxygen sensors read between 2.6 and 3.4V when we use enhanced OBD II on an OTC Genisys. Why is the Bank 2 sensor stuck at 3.3V? We’ll see in a moment.

FIGURE 2: This Chrysler wiring diagram, courtesy of Mitchell 1 ProDemand, shows how the power/return wire runs through the variable capacitor to the sensor and exits as the signal wire to the PCM.
<p>FIGURE 2: This Chrysler wiring diagram, courtesy of Mitchell 1 ProDemand, shows how the power/return wire runs through the variable capacitor to the sensor and exits as the signal wire to the PCM.</p>

Why is this? Instead of being a zirconium dioxide oxygen sensor, these 5V sensors use titanium dioxide.

It is probable that this fact has skipped many technicians’ notice, simply because they do not look at Chrysler oxygen sensors very often. Chryslers do not like throwing system lean DTCs, even when they have massive vacuum leaks.

The way titanium dioxide oxygen sensors on Chryslers work is that they receive 5V of power from the PCM from what is called an “O2 return” wire. The Bank 1 and Bank 2 sensors share the same O2 return wires and each individual sensor has a “signal wire” which in reality is the ground wire (refer to Figure 2 for a Chrysler wiring diagram, courtesy of Mitchell 1 ProDemand).

FIGURE 3: On this 2014 Dodge Charger 3.6L the front oxygen sensor works just like a traditional one, but the voltages reflected in generic OBD II, between 0V to 1V are inaccurate.
<p>FIGURE 3: On this 2014 Dodge Charger 3.6L the front oxygen sensor works just like a traditional one, but the voltages reflected in generic OBD II, between 0V to 1V are inaccurate.</p>

This shows how the power/return wire runs through the variable capacitor internal to the sensor and exits as the signal wire to the PCM.

The changes in oxygen in the exhaust create a voltage drop in the sensor, which the signal/ground wire sends as a voltage to the PCM.

FIGURE 4: The rear oxygen sensor on the same vehicle has a steady voltage of around 685mV according to generic OBD II. We know this is inaccurate, but we know that on traditional oxygen sensors, a steady reading of such a voltage would reflect high catalytic efficiency. After going WOT, the voltage drops precipitously as the fuel mixture is leaned out during the throttle snap.
<p>FIGURE 4: The rear oxygen sensor on the same vehicle has a steady voltage of around 685mV according to generic OBD II. We know this is inaccurate, but we know that on traditional oxygen sensors, a steady reading of such a voltage would reflect high catalytic efficiency. After going WOT, the voltage drops precipitously as the fuel mixture is leaned out during the throttle snap.</p>

The PCM then interprets these voltages to know what air/fuel mixture or catalytic efficiency is. Just like on a standard oxygen sensor, these sensors “switch” up and down in an identical fashion.

Instead of switching between 0V and 1V, Chrysler specifies that they switch between 2.5V and 3.4V. When using OEM Enhanced data on a scan tool, or a labscope, this is how it will appear.

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