The Oxygen (O2) Sensor can be used as a diagnostic tool on today's computerized vehicles. In order to use this "built-in" diagnostic aid, we must first understand how it works.

How The O2 Sensor Works:
 
The O2 sensor can be considered a small battery that has an operating range of 0 to 1 volt when fully warmed up to 600°F. Its voltage depends on the amount of oxygen in the exhaust stream.
 
All O2 sensors are vented to the atmosphere which contains Approximately 21% oxygen. The exhaust of a gasoline Powered engine typically contains up to 2% oxygen. The Sensor's output voltage depends on the oxygen content of the exhaust stream. That is, if the exhaust has 2% oxygen, it is lean. This produces a low voltage, below .3 volt (300 millivolts). If the exhaust has near 0% oxygen, it is rich. This produces a high voltage, above .6 volt (600 millivolts). These voltages are sent to the computer and it reacts by adjusting the air/fuel ratio. This is commonly known as the O2 feedback system and when this system is operating it is said to be in "closed loop". When it is not operating, meaning the computer is not reading and responding to the oxygen sensor, it is said to be in "open loop".
 
Keep in mind that the computer uses all the sensors to control timing, fuel mixture, and emission systems. The O2 sensor as an input is used by the computer to keep the mixture as balanced as possible. When the air/fuel ratio is "balanced" it is at 14.7 parts of air to 1 part of fuel by weight. That means that for every pound of gasoline the engine burns, it will need 14.7 pounds of air. Keep in mind that oxygen occupies only 21% of the total air volume needed by the engine. The term "stoichiometric" is the term referring to the point at which the catalytic converter can be at its maximum efficiency when converting the three major pollutants (CO, HC, NOX) into harmless emissions (CO2, H2O, N, H).
 
The computer can only use the sensor's output information under certain conditions. First, the sensor must be hot to produce a normal signal. (600°F). This is why most sensors today have built-in heaters to counteract the cooling effects of prolonged idling and to achieve closed loop mode sooner during warm-up. Heating the sensor also keeps it cleaner and extends its life considerably. The heater usually gets voltage from a constant "key-on" source like the fuel pump relay or a fuse. This is what the second and third wires are for on today's sensors. By the way, the late model Chrysler products are now using 4 wire O2 sensors. The four Wires are; O2 sensor output, O2 sensor ground, 12 volt heater feed, and heater ground. On 3 wire sensors, the O2 sensor grounds through its case and doesn't require a separate ground wire.
 
Secondly, the computer is programmed not to go into closed loop operation until the coolant temperature sensor tells the computer the engine is warmed up. If the system tries to go into closed loop too early in the warm-up period, the leaning effect of the system would cause driveability problems and pollutants.
 
Thirdly, the computer is also programmed to ignore the O2 sensor at near wide open throttle conditions. Maximum power requires maximum enrichment.
 
Also, some manufacturers have a built-in time delay. For example, on some GM models, closed loop is delayed for 1 to 2 minutes every time the car starts. This allows engine stabilization to take place before the engine goes into closed loop.
 
We can then conclude from the open loop conditions above that O2 sensor feedback is used when the engine is warmed up, at Idle, and at part throttle (cruise) conditions.
 
In order to read the O2 sensor, most computers send out a certain voltage to the output terminal of the sensor. This is typically around 450 millivolts. Since we know that the sensor sends low voltage (under 300mv) when a lean condition is present and a high voltage (over 600mv) when a rich condition is present, the computer can count the number of times the sensor crosses the 450mv mark. Cross-counts are the number of times an O2 sensor crosses 450mv. A scanner can "look" at this for you.
 
Even though you can't see the number of cross-counts without a scanner, you can use a digital voltmeter to watch the open/closed loop system operate. Just connect your meter as Illustrated above, while the O2 sensor is still connected, and start-up the car.
 
Caution: Do not ground the output wire of the sensor. This can damage the sensor and your readings will be erroneous.
 
When the car starts (cold), you should see approximately .450 Volt (450mv) on the O2 output wire. This reading often varies slightly. The system is now in open loop.
 
After a few minutes (less if the engine is warm or the O2 Sensor is heated), this reading should begin to fluctuate. You will see changing numbers ranging from near 0 volt to near 1 Volt. If these readings occur, everything is ok. The O2 sensor's output should vary relatively quickly. A lazy sensor, would show up here and the readings will vary slowly. If the readings don't start to vary (stay in open loop), you now are aware of it and can begin to look for the reason.
 
How Can We Use The O2 Sensor As A Tool?

Let's now see how this information can help us diagnose problems. This "tool" can be especially helpful if another sensor is out of normal range but not enough to trip a trouble code.
 
Since engines still operate on the same principles as they always did, we must keep in mind that mechanical problems can occur. We must remember to consider compression, cam timing, ignition timing, fuel quality, filters, and exhaust restrictions whenever we come across driveability problems.
 
Always begin diagnosing a driveability problem in a computerized vehicle by retrieving the codes first. Hard codes, (check engine light on) will give you an obvious starting point. Soft codes (engine light was on intermittently but is not on at the moment) will also give you a good start for diagnosis. Keep in mind that a code is an indication of a failure or the result of a failure. For example an O2 sensor may be constantly showing a rich condition which will trip a code. The sensor may or may not be the problem. The problems, may be in a system that could cause a rich condition. The same goes for lean O2 sensor readings.
 
Oxygen sensors can set trouble codes for various reasons. An open sensor wire can set trouble codes. If a sensor stays lean or rich for a long period of time, lean or rich codes may set. In either case, the sensor may be at fault. Before condemning the sensor, there are some very important checks that should be made.
 
For continuously lean O2 sensor readings:
 
1. Check sensor output wire for possible grounding. A ground will cause a false lean signal.
 
2. Check the MAP sensor for proper vacuum to voltage output. A high vacuum signal will cause a lean ecu reaction. (Don't forget to check manifold vacuum first!)
 
3. Clogged injectors can cause a false lean condition. A cleaning may solve the problem.
 
4. Water contamination will cause a lean condition.
 
5. Low fuel pressure will cause lean conditions at any rpm or load range. Be sure to check pressure at all driving modes.
 
6. Exhaust leaks, especially near the sensor can pull in air and cause a false lean reading.
 
7. Check for proper air injection system operation. The air pump should not direct air to the exhaust ports during closed loop operation.
 
For continuously rich O2 sensor readings:
 
1. Check the fuel pressure. High readings will cause rich conditions.
 
2. Leaking injector(s) will cause rich exhaust.
 
A leak down test and/or a power balance test can usually reveal the leaker.
 
3. A contaminated or malfunctioning canister purge system can very easily put uncontrollable amounts of fuel into the intake manifold. Simply disconnecting the vapor hose can reveal this as your problem system.
 
4. Check vacuum to voltage readings at the MAP sensor. A low MAP output will cause a rich ecu reaction. (Don't forget vacuum readings again!)
 
5. A false tps signal can cause the system to go rich if the Ecu sees a high tps output. Check tps readings at idle and for a smooth rise to wide open throttle.