Okay, so when is an O2 sensor "bad"? If you work in a shop where your customer can walk in on any given day and slap a failed emissions test form onto your counter, then a bad O2 sensor could be one that is not working as it should and directly affecting the CO emissions that spew from your customer's car. But what about the rest of you who don't face any failure forms during the course of a day? Will you ever be able to sell your customers an O2 sensor simply because it may be worn out?

Consider these figures from Robert Bosch Corp.: If you have two identical cars that drive 12,000 miles in one year, but one has a new O2 sensor and one has a worn-out sensor, the vehicle with the new sensor could have a 10% increase in fuel efficiency, resulting in a fuel savings of $87 per year. Now, this assumes $1.30 per gallon for fuel, but, let's face it, gas is closer to $2 per gallon this summer. Extend this to 50,000 miles and you have a savings of $362. Could these numbers entice customers to replace their O2 sensors as routine maintenance at, say, around 30,000 miles? Many imports used this mileage as a replacement interval, but most domestic manufacturers don't.

This certainly doesn't apply to OBD II vehicles, which can have up to four sensors per car, and with the advanced monitors in place, replacing O2 sensors on these vehicles as preventive maintenance isn't advisable. But, the majority of the powertrains running down the road are still OBD I (until about 2005), so these should be the target traffic for increased O2 sensor sales.

Another study commissioned by Bosch suggests that degraded O2 sensors are a major cause of harmful hydrocarbon (HC) and carbon monoxide (CO) emissions, especially in fuel-injected vehicles. The study also found that replacing all degraded O2 sensors nationwide would reduce total HC and CO emissions more than all other automotive repairs combined - HC would be reduced by 23%; CO by 33%. The results of the study show that more than half of all vehicles failing HC or CO emissions have degraded oxygen sensors.

The I/M 240 Emissions Test As A Fuel Efficiency Indicator
Ever wonder how much authority an O2 sensor has over fuel delivery?
The red line indicates the CO emissions over the course of the test. This Jeep failed our Colorado test for CO and HC. CO failed at 65 GPM on a 30 GPM standard. After systematic diagnosis, I found a sluggish O2 sensor that failed the response and calibration test. After replacement, the Jeep returned to the inspection lanes and turned in the numbers shown in Figure 3. It passed at 0.8 GPM on a 30 GPM scale! (Deep breath in&#8230 Wow, the air is cleaner already!

Good Sensors Gone Bad
Sensors can fail due to a variety of reasons: silicone contamination from antifreeze or non-sensor-safe silicone sealer, lead from leaded fuel, ash from excess oil burning and carbon fouling from a rich mixture.
Don't use a silicone sealer for making engine gaskets unless it has this warning or is recommended by the vehicle manufacturer. Sensors can also read below 0 volts if the thimble becomes cracked, or if the fresh air vent hole is plugged.

Many older sensors have a vent hole in the side of the sensor for reference air. These could easily become plugged with mud or splattered squirrel parts. For this reason, newer sensors "breathe" through their wire connectors, allowing air to travel between the wire and insulation. Grease in these sensor connectors can cause the reference oxygen to be lost.

One Size Doesn't Fit All
Aren't all zirconia sensors the same? Why are there so many different part numbers for zirconia sensors?

First, the physical design of the sensor is critical. The number and design of the slits in the sensor is crucial. Some have only a few slits, such as turbocharged engines, and others have many slits. Also, some sensors have clockwise slits, others have counter-clockwise slits. If this design is reversed, exhaust flow across the sensor will be altered.

Secondly, many late-model PCMs have a degradation factor built into them that make up for degradation of the sensor with increased frequency. Bill Sauer of Autoline Telediagnostics writes that replacing the original sensor with one that has a different degradation factor will affect the calibration factor of the PCM. This is like bowling with a new ball that weighs more and has the finger holes drilled differently; you'll have a hard time calibrating your brain to the new ball, and your game will suffer

The spinell layer of each O2 sensor determines its degradation factor. This layer is sprayed onto the zirconium to protect it from contamination in the exhaust stream. Some sensors, such as Japanese sensors, have a smaller micro porosity of the spinell layer, so they are more apt to plug up sooner.

Engineers consider these various characteristics to develop a sensor with unique response times. The aftermarket usually develops a sensor that meets the most stringent of the possible standards. This allows them to cover more applications with one part number. As with most things in life, let the buyer beware! According to Chris Chesney of Diagnostic Technician Education Consultants (DTEC), one of the most common faults of aftermarket sensors is the small sample holes in the thimble. If the replacement sensor has smaller holes than the original sensor, question its use.

Oxygen Sensor Testing
On a recent iATN tech night, I typed in the question: "How are all of you guys testing O2 sensors these days?" First, I received the answer of "frequency."

I have seen the following specs regarding the switching rate of O2 sensors: Feedback carburetors switch at about 1 Hz (once per second) at 2,500 rpm, throttle body injection systems switch at 2-3 Hz at 2,500 rpm, and multiport injection engines switch at 5-7 Hz at 2,500 rpm. Knowing this, a tech can use a DVOM set to Hz and simply measure the switching rate of the sensor to determine if it's working properly. The problem with this test method is that we can't tell how high or low the sensor is reading, or if it's biased lean or rich.

The second answer I heard online was to "test it for rate-of-change." I call it the response and calibration test, and I agree with this method. To test an O2 sensor for response and calibration, set a digital storage oscilloscope time-base to 1 second-per-division, and the voltage to 200 millivolts-per-division. Next, induce propane into the air intake, or simply snap the throttle open to force the mixture rich. The O2 waveform should show up as a straight line on your scope. If you use your cursors, the waveform should be able to switch from lean (100 millivolts) to rich (900 millivolts) in less than 100 milliseconds.

If your O2 sensor can fall below 100 millivolts, climb above 900 millivolts, and do it in less than 100 ms., then that sensor is good.

"This is the best way to confirm the O2 sensor," says Chesney. He continues to say, "If you're going to use the O2 sensor as a diagnostic tool, you first must check the calibration of your tool by performing the response and calibration test."

If there are two sensors on opposite banks of the engine, they should mirror each other on the scope. My favorite story regarding dual O2 sensors comes from my friend, Scott, at a local Ford dealer. He had a Ranger truck come in with a complaint of a severe power loss after an engine replacement. Scott did all of the usual checks: timing, trouble codes, backpressure, etc., but couldn't find a cause. Finally, he hooked his lab scope up to both O2 sensors and went for a test drive. When one sensor went rich, the other went lean; it was as if each sensor were trying to adjust the fuel mixture on the opposite bank! Further investigation revealed that the O2 sensors had identical connectors, wire length and colors. Swapping the wires returned the engine to proper fuel control.

When Is An O2 Sensor Active?
Chesney says that he asks this question in his training classes quite often, and receives 600° F as a response. He makes a point to tell techs that O2 sensors are "active" at 600°, but are "accurate" at 800°.

Most late-model O2 sensors are three- or four-wire, heated sensors, so they can reach 800° faster and reduce high-polluting start-up emissions. When these sensors' heaters fail on OBD II vehicles, the MIL is illuminated and a trouble code is stored. I recently found a generic code PO154: "HO2S1 Circuit Remains At Center (Bank 2, Sensor 1)" in a 1996 Dodge Intrepid.

Out With The Bad Sensor, In With The Good...
When an O2 sensor is found to be defective, where do you buy yours? The safest bet is to replace it with a dealer part, or an aftermarket one that doesn't require slicing of the old pigtail onto the new sensor.

If splicing is required, follow the instructions that come with the sensor, but keep this in mind: many sensors have shielded leads that travel to ground. Using the incorrect procedure when attaching the old lead to the new sensor can result in the grounding of the O2 signal, and then guess what happens to the fuel control!

I was raised to always solder and heat shrink my connections, but current thinking gives the green light to using butt connectors on these jobs, as long as heat shrink is used over the butt-splice. Don't do what one do-it-yourselfer did in Figure 10; he twisted the wires together and wrapped it with electrical tape! Please remember: Some oxygen sensors use the wiring (actually sheet on wiring) as a channel for reference oxygen. If you cut off the connector, solder and shrink-wrap, you may be blocking reference oxygen.

Titania Oxygen Sensors
Titanium dioxide (Titania) O2 sensors work differently from zirconia O2 sensors in several ways. A titania O2 sensor is a variable resistor; its resistance changes depending on the amount of oxygen present in the exhaust. Titania O2 sensors contain a resistor that changes to vary the reference voltage applied to it, creating a signal to the computer. The sensor is fed a reference voltage (usually 1 volt, but the pre-1991 Jeep 4.0L uses 5 volts), and returns a lower voltage. The voltage it returns is proportional to the oxygen content of the exhaust. Titania O2 sensors act like thermistors. Instead of changing resistance in relation to temperature, however, a titania O2 sensor changes its resistance chemically in relation to the difference in oxygen in the exhaust. Exposure to ambient air as an O2 reference is not required, so air holes won't plug up, like the one on the zirconia sensor. That's why DaimlerChrysler went with these on their Jeep products.

There are two types of titania O2 sensors: 0-5-volt and 0-1-volt systems. The 1986-90, 4.0L Jeep systems use a 5-volt reference sensor; others use a 1-volt reference sensor. All but the Jeep 5-volt titania O2 systems generally follow the same performance specs as zirconia sensors. Zero- to 1-volt titania O2 systems are used on some 3.0L Chryslers (Mitsubishi V6 engines), Eagle Summit, 1986 and later Nissan 300 ZX and Stanza 4WD, 1982 and later Nissan Maxima and Sentra, late 1983 and later Nissan D21 trucks, and some 1988 and later Toyotas, such as the 4-Runner. The 1986-90 4.0L Jeeps have two unique features: 1. The sensor switches from 0 to 5 volts instead of 0 to 1 volt, and, 2. Rich and lean voltages are opposite from other O2 sensors. Rich exhaust mixtures create low voltages and lean mixtures create high voltages. The response time specifications are generally the same for titania and zirconia sensors.

Catalyst Monitors
OBD II vehicles have their own on-board, full-time catalyst efficiency monitors that use pre-catalyst and post-catalyst oxygen sensors to monitor the efficiency of the converter. The upstream HO2S1 is used to detect the amount of oxygen in the exhaust gas before the gas enters the converter. A low voltage indicates high oxygen content (lean mixture) and a high voltage indicates low oxygen content (rich mixture). When the upstream HO2S1 indicates an abundance of oxygen in the exhaust, the converter stores it to oxidize HC and CO gases.

As the converter absorbs the oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream HO2S2 will indicate limited activity during this condition.