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Copyright © 1999 Adams Business
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HEGO Trip:
Oxygen Sensors Revisited
After two dozen years, the whole truth
by Bob Freudenberger 
A HEGO can have up to four wires, the black one for the signal. This is an
aftermarket direct-replacement unit ($35).
We remember being intrigued and astonished back in '76
when the oxygen sensor first appeared as the cornerstone of the Saab/Bosch Lambda-Sond
system. The industry was just becoming accustomed to electronic ignition, and this step up
into closed-loop engine management was staggering. It knocked the service business for a
loop, and some of us haven't fully recovered yet. The motoring public, of course, is still
blithely unaware of the whole concept--customers are commonly shocked to hear that their
cars have a computer aboard, or that there's such a thing as a sensor that gives it input
on the air/fuel mix (see sidebar).
Besides being amazed at the idea of on-board exhaust analysis by means of what's
variously called an oxygen, 02, EGO, or HEGO sensor, or even a
Lambda probe by those who like the European touch (the Greek letter lambda represents the
ideal stoichiometric 14.7:1 air/fuel ratio in engineering jargon), we were dubious at the
time about the potential longevity of a sensitive component located in that horrendous
blast of hot gases. How long could it possibly last?
That fear proved unfounded. Early on, the carmakers recommended replacement every 15K
or 30K miles. Experience has shown that many specimens are still fine, thank you, after
several times that, and regular retirement intervals were lengthened to 60K, then
eliminated altogether. "Many" is the key word, though. Lots of them do indeed
die prematurely.
A study conducted for CARB (California Air Resources Board) some years ago stated that
70% of fuel injected vehicles that fail the state's emissions test have bad 02
sensors. Also, a faulty one can cause fuel economy to decline by 10-15%, costing an
unheedful motorist maybe $100 a year, perhaps melting down the cat in the process.
Platinum-coated zirconium
There are really two basic types of mixture monitors, zirconium and titanium.
Introduced in 1986, the latter is found only in a small percentage of the vehicles on the
road, so we'll concentrate on the former here, and hit titanium later.
A basic zirconium oxygen sensor comprises a steel housing with a hex and threads, a
louvered shield over the tip, and a hollow cone-shaped internal element made of zirconium
dioxide (Zr02, a ceramic material), which is coated inside and out
with a thin layer of micro-porous platinum. The outer layer is exposed to the exhaust
stream, while the inner layer is vented to the atmosphere and attached to a wire that runs
to the PCM.
It's actually a galvanic cell--think of a flashlight battery. The zirconium dioxide
acts as the electrolyte, and the platinum layers serve as electrodes. Once the Zr02
reaches about 600°F, it becomes electrically conductive and attracts negatively charged
ions of oxygen. These ions collect on the inner and outer platinum surfaces. Naturally,
there's more oxygen in plain air than in exhaust, so the inner electrode will always
collect more ions than the outer electrode, and this causes a voltage potential--electrons
will flow.
The concentration of oxygen in the exhaust stream determines the number of ions on the
outer electrode, hence the amount of voltage produced. If the engine is running rich,
little oxygen will be present in the exhaust, few ions will attach to the outer electrode,
and voltage output will be relatively high. In a lean situation, more oxygen will be
present, and that translates into more ions on the outer electrode, a smaller electrical
potential, and less voltage. Just remember "L=L" for Lean=Low.
The voltage is always small, never exceeding 1.3 volts (or, 1,300 mV) or so, with a
typical operating range being between 100 and 900 mV. But it's enough for the computer to
read. If it receives a sensor signal of less than about 450 mV, it recognizes a lean
condition, and if it gets more than that amount of voltage, it registers a rich condition
(it's one or the other, there's no in-between). Either way, it instantly corrects by
adjusting the injection pulse width or feedback carb duty cycle.
You may have heard the term 'dithering' applied to 02 sensors.
The dictionary will tell you that a "dither" is a state of agitation or
indecision. As a verb, it's a pretty good description of how this sensor's signal is
supposed to be continuously switching from rich to lean and back.
No heat, no go
Remember the 600°F temperature mentioned above? Good, because it's important. Oxygen
sensors simply will not produce voltage until they're hot, and many perfectly good units
are replaced because some technicians test them when the engine is first started, or after
it's idled for a while, which allows the unheated type to cool off.
With a cold sensor, the computer realizes it's not getting a signal and will operate in
open-loop, holding the mixture at a fixed setting. Since good fuel efficiency and minimum
exhaust emissions can only be had during closed-loop, it's desirable to get the sensor hot
as soon as possible after the engine is started, and to keep it there while idling, so
late-model oxygen sensors are equipped with an electrical heating element.
Wires
While your basic sensor will have one wire, you'll also see specimens with two, three,
or four. In most cases, two means it's an ordinary unheated unit. The extra wire just
provides a more dependable ground than thread contact in the manifold or head pipe.
Although the carmakers and OE sensor manufacturers warn us that we're liable to get low
voltage/lean readings if we substitute a one wire universal, it's done all the time in the
real world of service with varying degrees of success. If you take this route, make sure
the threads it goes into are in decent shape.
Three leads indicate a HEGO (Heated Exhaust Gas Oxygen) sensor. Although the signal
wire is typically black, it can be hard to tell by just looking which of the other two is
ground and which is juice to the heating element, so a wiring diagram would be helpful.
The fourth wire? Simply a dedicated ground for the signal portion.
Numerous
If all zirconium oxygen sensors work in pretty much the same way, why are there so many
part numbers? To begin with, the replacement should have the right number of wires, as
just mentioned. So there you have four right off, but it actually works out to more than
that because of the different styles of pigtail connectors various carmakers use in their
harnesses. Some aftermarket suppliers list a couple of dozen direct-fit numbers all with
different connectors.
But there are other, more subtle considerations. For instance, there's the spacing of
the louvers in the business end. Most common is the wide-slot type, but there's the
narrow-slot variety, too. In unheated sensors, its purpose is to protect the ceramic from
thermal shock in applications that put the sensor close to the exhaust ports by making it
heat up more gradually. If you were to install a narrow-slot unit in a car that requires
the wide-slot type, it would take a long time to heat up to operating temperature, and may
keep throwing the car into open loop at idle because it's not staying hot enough to work.
There's really no problem using a wide-slot universal in a narrow-slot application,
however. With heated units, the situation is reversed. Narrow slots are used to keep the
sensor from cooling off.
A mention of the waterproof feature that's present on numerous recent versions is
appropriate here. All it means is that the reference air isn't picked up through a vent
right at the sensor itself where water and other contaminants are plentiful. Instead,
there's a sealed sleeve of insulation that runs up into the harness, or even all the way
to the PCM, before it opens to the atmosphere. Since only a tiny amount of reference air
is needed, enough will flow between the gaps in the stranded wires inside the insulation.
You can get into trouble if you solder these cables because it wouldn't take much flux,
solder, or melted plastic to block those essential gaps.
Substance abuse
Several things can wreck an oxygen sensor. Mechanical damage in the form of a broken
element or wire certainly happens, but the most common killer is contamination. Lead,
carbon, metals from motor oil additives, and silica (from high-volatile RTV silicone
sealants or anti-freeze--expect 02 sensor problems whenever you
replace a blown head gasket) in the exhaust can all coat that precious platinum and make
the unit sluggish or altogether inoperative. There have even been cases of sensor
poisoning from gasoline with a high silicon content, 500 ppm when 4 ppm is enough to cause
trouble. By the way, silica (Si02) appears as a fine white powder on
the shield.
Deposits on the exhaust-side electrode increase voltage output, giving a false rich
signal. This drives the system lean. But these coatings aren't always the kiss of death.
Try running the engine at 3,000 rpm for a few minutes, or take a drive, then retest. You
may have burned off whatever was interfering with juice generation. On the other hand, if
silica or metal deposits get hot enough, they'll melt into a coating that can never be
removed.
For some reason, most people have never even considered the possibility of
contamination of the electrode on the reference air side, but it's a problem nevertheless.
Typically, it comes from the smoke given off during the deterioration of silicone rubber
seals or insulation, or from aerosol wire-drying chemicals, cleaners, etc. used under the
hood. Blocking the reference air lowers the unit's voltage potential, making it send a
false lean signal that drives the mixture rich, possibly cooking the catalyst.
Likely complaints
Symptoms of a lazy or dead oxygen sensor are surging, hesitation, poor overall
performance, lousy gas mileage, and rough idling (all with a warm engine--never blame the
sensor for a cold driveability or performance problem), a failed state emissions test, and
a plugged cat. As you know, it's an unfortunate fact that those problems can be caused by
lots of other conditions besides a malfunctioning Lambda probe. So, you've got to do some
testing.
Where equipment's concerned, the obvious first choice is your scan tool. Looking at 02
voltage, cross-counts, and fuel trim is essential. Most techs, however, like to back up
that data with direct checks. A quality DMM (Digital Multi-Meter) with a voltage bar scale
or a lab scope will work fine here, but we're also happy with several job-specific testers
we've tried. Typically, these have LEDs that tell you activity and voltage, and switches
that allow you to stay in closed loop while monitoring, isolate the sensor from the
computer for open loop checks, or send the PCM either a rich or lean signal.
By the way, every carmaker cautions us to use only meters or testers with high
impedance. That's because there's a possibility of current flow breaking the bond between
the zirconium dioxide and the platinum, or blowing a channel through the sandwich. And if
you're checking it in parallel, an extra ground may skew the signal.
If the sensor is easily accessible, you might want to take it out for inspection. Shake
it to listen for rattles that indicate a broken ceramic element, gives it a careful visual
exam, then hook a DMM across the pigtail and the shell and heat the tip with a propane
torch (use the blue part of the flame). When it turns to a dull red, look for relatively
high voltage, then move the flame away, which should make the reading drop.
Robert Bosch gives a very straightforward procedure for testing with the sensor
isolated from the PCM. Start by warming up the engine, then disconnect the sensor's
pigtail from the harness and attach it directly to your meter or tester. To check rich
response, hold 2,500 rpm, and add propane to the intake until speed drops by 200 rpm. Or,
pull and plug the vacuum hose to the fuel pressure regulator, which will increase psi and
richen the blend. You should see the reading jump to 900 mV or more. If reaction is slow,
or that voltage is never reached, try running it at 3,000 rpm for a few minutes, then
check again. No improvement means you buy a new sensor.
Then, test lean response. Introduce a small vacuum leak, say by removing an accessory
hose, and watch the reading. If it drops to .2V or lower in less than one second, the
sensor's okay. If it falls sluggishly, or you never see it get down to .2V, give it the
3,000 rpm treatment and try again, but it's probably time for replacement.
Reconnect the pigtail to put the sensor back in touch with the PCM, then tap your meter
into the signal wire, maintain 1,500 rpm, and you should see rapidly changing readings
that average somewhere around half a volt as the computer keeps adjusting the blend.
Deciding whether or not response is slow enough to justify replacement requires some
judgment. A common rule of thumb for minimum activity is eight trips across the rich/lean
line in ten seconds, and sometimes you can find specs for cross-counts.
Sending the PCM a substitute 02 sensor signal will let you know
how--and if--the system responds. While some factory manuals say you can use an ordinary
dry cell to inject a rich signal, a sensor simulator would be a lot more helpful. If
varying the substitute signal changes something (say, injector pulse width, loop status,
ignition timing, idle speed or quality, etc.), you've learned two things: The wiring and
connections between the sensor and the PCM are okay, and the computer itself is
operational because it's responding to this input. Ergo, the sensor becomes your prime
suspect.
OBD
Will pre-OBD II self-diagnostics help you nail a weak 02 sensor?
Not really. Sure, the program monitors this critical input, but typically it's only
looking for opens or grounds in the circuit, an always rich/always lean situation, or
sensor cool down. It won't recognize a lazy specimen. So, this sensor is a likely cause of
driveability complaints even if no codes are present.
Things are different with OBD II vehicles. Not only is there a HEGO both upstream and
downstream of the catalyst, but two types of tests are run on them. One monitors the
sensor's activity, and the other checks the sensor's electrical heating element.
An oxygen sensor can fail the activity monitor for slow response rate, which is
sometimes called a "big slope" for the way the wave looks on an oscilloscope.
Or, it can fail for having too small a change in voltage output from rich to lean and vice
versa.
We'll use Ford as our example of how a HEGO response test is done. After the system
achieves closed loop, and during steady state speed and load in the 30-50 mile-per-hour
range, the test forces fuel control changes at high frequency. In other words, it
superimposes a square wave on top of the regular injector pulse. This switches rapidly
between 10% rich and 10% lean. Can the HEGO respond to this high frequency, or has it
become too sluggish and lazy? And, can it produce big enough voltage swings? If it can't
satisfy both standards, it fails.
Since any oxygen sensor is only accurate when it's hot, the HEGO heater monitor is
intended to make sure the sensor is properly heated. In the Chrysler approach, the heating
element itself isn't tested. Instead, the change in electrical resistance that the
sensor's output circuit experiences at various temperatures is watched.
After the ignition has been turned off, the PCM sends a five-volt signal to the sensor
every 1.6 seconds. As the sensor cools down, its resistance increases, changing the
voltage, which the PCM reads as temperature. Once the sensor is cool enough, the PCM
commands a relay to complete the circuit to the heating element, then it watches as the
sensor gets hotter and hotter. If it sees the proper voltage for several pulses, it passes
the sensor.
Titanium
The titanium oxygen sensor is a whole different animal (it's often called
"titania," but that's the name of the fairy queen in English folklore, not that
of the metallic element). Nissan is the biggest user. In '93, for example, every model
except the Maxima and the V6 pickup had titanium sensors. There's also a sprinkling of
others on the road. AMC used it on Jeeps from '87 to '90, but when Chrysler bought that
company, all engines were switched over to Mopar's own corporate system, which requires a
zirconium sensor. Some Toyotas have had it, too, such as the '89 Corolla GTS.
Instead of generating voltage, its resistance changes dramatically when the oxygen
content of the exhaust reflects stoichiometry, so it's used to modify a reference signal
(typically one volt) from the computer in much the same way as a coolant temperature
sensor. What happens is oxygen blocks the passage of electrons through the Ti02
element, and resistance rises sharply as the mix goes lean.
It's much less susceptible to lead poisoning than zirconium, but that's not a
consideration anymore. On the other hand, it can't tolerate anti-freeze, and it needs to
be kept at a very stable temperature in order to produce an accurate signal, so all
specimens are electrically heated.
How can you tell whether or not the sensor you're looking at is titanium? In the case
of Nissan, it'll have 12mm threads instead of the standard 18mm. Sometimes, its wire will
be color-coded red. In every instance, there's no atmospheric vent.
Etcetera
We'll conclude with some miscellaneous points:
- An oxygen sensor is an averaging device that responds to the leanest cylinder--it can't
differentiate among them. If one injector is clogged, it'll pump enough 02
to fool the computer into thinking there's an overall lean condition. So, the PCM will
fatten up the mix, perhaps increasing emissions.
- Whenever you replace an oxygen sensor, please make sure the threads are coated with the
proper anti-seize compound. Otherwise, the next guy who tries to remove it may have to
blast. There's a good chance the next guy will be you.
- To get mileage up and meet ULEV (Ultra Low Emissions Vehicle) standards, the oxygen
sensor is becoming more complex. By using zirconium's ability to "pump" ions in
a process we won't get into here, this sensor makes lean-burn engines with amazingly
efficient characteristics possible. Called the "wide range" or "lean"
oxygen sensor, it doesn't simply send a lean/rich toggle. Instead, it tells the PCM how
lean or rich the mixture is by providing useable output over a wide range of a/f ratios.
Selling 'em
All of us in auto service know that the mixture monitor enables
the engine management computer to maintain stoichiometry. That not only reduces fuel
consumption and combats air pollution directly, it also allows the three-way cat to do its
job of reducing NOx to harmless nitrogen and oxygen.
Fine, but your average customer hasn't got a clue about the whole
situation. When you tell him he needs a new 02 sensor, you get a
blank stare. If his catalyst has entered melt-down and you suggest that the oxygen sensor
was probably the cause, he might suspect that you're trying to rip him off.
As Bob Peppin, New Hampshire shop owner, '82 NAPA/ASE Technician
of the Year, and long-time Motor Service Technicians Advisory Panel member, tells us,
"Oxygen sensors should probably be changed at 50,000 miles, but nobody's doing it.
I'll bet a third of them out there aren't working right."
So, how do you get your patrons to have this needed maintenance
done before trouble sets in? First, take the time to explain this high-tech aspect of
their cars to them. Keep a diagram of inputs/outputs and some germane parts around.
Next, be positive. As one successful Texas shop owner says,
"We don't sell an O2 sensor. We sell better mileage, better driveability, cleaner
air. That happens to be controlled by the O2 sensor. We sell the benefits of replacing
it." Another Texan adds, "It's a win/win deal for everybody." Two other
things to mention are that most catalyst failures are due to a bad oxygen sensor, and that
the average motorist will probably save as much in gas annually as the sensor costs.
We believe in checking 02 sensors whenever
you've got a car in for any underhood service from a tune-up to a valve job. You're
probably going to do a scan anyway, and this can prevent dissatisfaction with your work.
Peppin says simply, "If it's slow, I tell the customer he needs one and this is the
price, period."
By the way, Peppin has a nice agreement with the people at his
local Meineke muffler shop. Whenever they replace a cat, they request that the customer
take the car to Bob's shop to have the oxygen sensor checked. This saves them warranty
returns and brings some work into Peppin's garage. ms
Copyright © 1999 Adams Business
Media, Inc.
All Rights Reserved. Reproduction Prohibited.
Would You Like A Reprint of
this Article?
CLICK HERE!
Please Note: some pictures or diagrams
are only
available through the printed media.
From the September 1999
issue of Motor Service.
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