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02 Sensors and Black Tape

Written by PFM Staff

When the amber “Check Engine” light, or “Service Engine Soon” light, or “Malfunction Indicator Light” (MIL) comes on, all patrol officers know this means the oxygen (O2) sensor has gone bad. They also know it is no big deal.

They know it is perfectly okay to keep driving because they are told to keep driving every time they mention the MIL light to someone in fleet maintenance. And no self-respecting traffic officer is going to go through the hassle of swapping cars, or use a dreaded “pool” car, just because the MIL light comes on. I have driven for many shifts with a MIL that flashed during acceleration and burned steady when idling or cruising...and I got nearly 18 mpg while writing a book of tickets.

Even still, the MIL light can be distracting. The steadily burning lights on the dash are not so bad. You can adjust the tilt wheel to block the light from being seen. Or sometimes a good 10-and-2 hand position on the steering wheel blocks the light. But the flashing MIL lights are so irritating and so hard to ignore that the only practical solution is to cover the MIL light with black tape.

The reality, however, is the so-called “Oxygen Sensor” light is a bit more important than that. Both a steady and flashing MIL light should be treated a lot more seriously than most officers—and some fleet managers—do. The MIL light does not always mean the O2 sensor is bad. In many cases, it means the sensor is good, and doing exactly what it was designed to do.

What Is The O2 Sensor?

The oxygen sensor is a chemical-driven, electrical generator. The O2 sensor compares the amount of oxygen in the exhaust with the amount of oxygen in the outside air. The outside air has about 21% oxygen, while the engine’s exhaust has about 2% oxygen. If the O2 sensor detects little or no oxygen during the comparison, a voltage is generated.

The theoretical output of the sensor is between 0.0 and 1.1 volts. The usual output range is between 0.2 and 0.7 volts. Any voltage above 0.45 volts is considered a rich exhaust, while any voltage reading below 0.45 volts is considered a lean exhaust.

A low oxygen reading (i.e., more than 0.45 volts, engine rich condition) means there was so much gasoline that nearly all of the oxygen was burned during combustion. When the engine is running rich, all the available oxygen is consumed during combustion, and the resulting exhaust gas contains little or no oxygen. The sensor sends a voltage between 0.45 volts and 1.1 volts.

A high oxygen reading (i.e., less than 0.45 volts, engine lean condition) means that all of the gasoline was burned without burning all the oxygen. When the engine is running lean, all the available gasoline is burned, but oxygen is left over and goes out in the exhaust. In this case, the sensor generates a voltage between 0.0 volts to 0.45 volts.

On vehicles with on-board diagnostics II (OBD-II), the engine has one O2 sensor per piston bank located ahead of the catalytic converter and one O2 sensor behind each catalytic converter. This allows the PCM to monitor both the engine combustion and the catalyst operation. The late model Ford CVPI with its dual exhaust has four oxygen sensors.

Three different kinds of heated O2 sensors exist, 1) ignition switch controlled for pre-OBO-II cars, 2) PCM relay controlled and 3) Fast Light Off PCM flow controlled. It is critical that the defective O2 sensor be replaced with correct one!

What Does The O2 Sensor Do?

The O2 sensor controls the engine’s air/fuel ratio. About seven times a second, the platinum-wrapped, zirconium-ceramic, semiconductor device checks the amount of oxygen in the exhaust gas. The PCM uses information from the O2 sensor to achieve optimum 14.7:1 air/fuel ratio under a wide variety of operating conditions.

The PCM also uses input from the coolant sensor, throttle position sensor, manifold absolute pressure sensor, mass airflow sensor, etc., but the O2 sensor is the main input for the fuel mixture. The MIL light, then, alerts the driver that something is malfunctioning in the most basic aspect of internal combustion engines!

If the O2 sensor reads rich (high volts, low oxygen), the PCM shortens the on-time of each injector pulse to reduce the amount of fuel being squirted into the engine. This makes the combustion go lean. As soon as the O2 sensor detects this lean condition (low volts, high oxygen), it increases the injector on-time.

The O2 sensor is constantly in a state of transition between high (rich) and low (lean) voltage. In fact, the more times the O2 sensor crosses the 0.45 volt mark (the O2 cross count), the better the sensor is working, and the better the rest of the PCM and engine are working. This closed fuel feedback loop produces crossing back and forth between rich and lean many times a second. The O2 sensor is the key sensor in this feed back loop.

A fully warm and fully functioning O2 sensor will not spend any time at exactly 0.45 volts. Only when the sensor is not up to temperature or if the circuit is not complete will the PCM pick up a steady 0.45 volts. In this event, the PCM remains in “open loop” operation and uses all the other sensors except the O2 sensor to determine fuel delivery. In the open loop mode, the engine runs slightly rich. That results in a loss of power, poor fuel economy and more exhaust emissions.

Most O2 sensors today are heated. A pair of wires runs current through the sensor to an internal heating element to preheat it, or to bring it up to operating temperatures faster. The faster the O2 sensor switches the PCM from open loop to closed loop, the lower the exhaust emissions. Without preheating, it takes about 2 minutes at engine speeds over 2,000 rpm to warm the O2 sensor and convert to closed loop operation. The internal heating element gets it the up to operating temperature in less than 35 seconds.

What Is Wrong?

The PCM will initiate an O2 sensor diagnostic code 1) if the sensor does not produce a voltage signal, 2) it if stays rich too long, 3) if it stays lean too long, 4) if it does not switch from rich/lean, i.e., stays in the center too long and, 5) if it does not switch rich/lean fast enough.

On the OBD-II cars (1996 and newer), the oxygen levels before and after the catalytic converter are compared to see if the converter is changing oxygen readings to reduce pollutants. An O2 sensor diagnostic code will also be initiated if the OBD-II sees little or no difference in the before and after oxygen readings, indicating that the converter is not working properly.

The O2 sensor is blamed for all kinds of fuel economy and drivability (and emissions) problems. The O2 sensor certainly can CAUSE drivability, fuel economy and emission problems. But (and here is the point) it can also DETECT problems caused by other faults in the drivetrain. When the MIL light comes on, assume the O2 sensor is NOT defective. Assume it really is a legitimate drivetrain problem. That is why often, when the O2 sensors are replaced, without a bit more diagnostic and detective work, the MIL light remains lit!

Exactly WHAT is wrong with the engine is not known. It could be minor. A frequent cause of the oxygen sensor code is, in fact, a loose or broken vacuum hose. This creates a lean mixture and causes the O2 sensor to try to compensate for a condition beyond its range of adjustment.

A PCV valve out of its hose or a split in a power brake vacuum supply hose will cause such a vacuum leak. It could be a loose gas cap. It could be broken wires to O2 sensor or poor grounding of the exhaust system or undercoating or grease on the O2 sensor connectors. Check other sensors and other parts of the wiring harness that may be shorted. The O2 sensor may be fouled by carbon from the engine running rich.

Any engine problem that allows unburned air to pass through the cylinders will trick the O2 sensor into a low reading. Any vacuum leak, a misfiring spark plug or ignition coil, a leaky exhaust valve, an exhaust manifold crack or gasket leak, a crack or hole in the exhaust system may allow enough air into the exhaust to trick the O2 sensor. It will respond by a call for more fuel. The MIL can also mean a compression problem. An engine that burns oil or develops a cooling leak may cause the O2 sensor to fail.

A defective mass airflow sensor or throttle position sensor may give bad data to the PCM and trigger the O2 sensor, which will in turn, illuminate the MIL. It could be a defective thermostat or engine temp sending unit. A bad coolant temperature sensor can also prevent the PCM from going into the “closed” feedback control loop. The PCM considers engine temperature (i.e., warmed-up engine) before going into the closed loop. Replacing the O2 sensor doesn’t fix any of these leaks! And the coolant leak, for one example, could make the officer really wish he had swapped his unit for a pool car.

O2 Sensor Goes Bad

O2 sensors do, indeed, wear out and need replaced. As the O2 sensor ages, it does not react to changing oxygen content as it once did. Its performance diminishes with age as contaminants accumulate on the sensor tip. This gradually reduces its ability to produce voltage. The O2 sensor gradually produces a lower voltage, signaling a higher oxygen, leaner condition. As a result, the PCM gradually produces a richer fuel mixture, resulting in loss of power, sluggish throttle response and poorer economy.

When the O2 sensor outright fails, it produces a lean (low volt, high oxygen) reading. This failure can be caused by old age, contamination or a bad wiring connection. This same condition can also be caused by an ignition problem, compression problem, vacuum leak or exhaust leak. If the engine burns oil or develops an internal coolant leak, the sensor may become contaminated and fail.

Sensor tip deterioration can be caused by a variety of substances that make their way into the exhaust such as silicone (from gaskets), sulfur and even some fuel additives. The O2 sensor can also be damaged by road salt, tar and oil.

The amount of oxygen in the ambient air, which the sensor needs for the comparison, literally seeps into the sensor from the outside. Undercoating, oil, antifreeze, road tar or other road-borne liquids or solids on the outside surface of the O2 sensor can block this seepage, and thus the comparison.

Preventative Maintenance

As the O2 sensor ages, it becomes sluggish in the time it takes to react to changes in the air/fuel mixture before it actually fails. During this time, the engine simply will not run as responsively as it used to run. That means you should not wait for the O2 sensor to actually fail before replacing it.

As the O2 fails completely, the default on most engines is to a rich fuel mixture. This is the same mixture used during the initial start on a cold engine. After a few moments, the sensor default is to a fixed, mid-range compromise mixture of air to fuel. With engines designed to have continuously variable air/fuel mixtures, this fixed mixture means the engine will actually never get the correct mixture. And the times that this fixed mixture is richer than needed, more gasoline is dumped into the catalytic converters, causing them to overheat.

The lack of a fault code, or illuminated MIL, does not necessarily mean the O2 sensor is working properly. A badly degraded and aged O2 sensor may operate well enough not to send a fault code to the PCM. It may operate well enough to pass a test with a scan tool or digital voltmeter, since most tests indicate a voltage change (amplitude of the wave) but not the frequency (change from rich to lean) or the response time.

Replacing sluggish O2 sensors will restore peak engine efficiency (throttle response), minimize emissions, maximize fuel economy, and most important, it will prolong the life of expensive catalytic converters.

The degraded performance before failure makes the O2 sensors perfect candidates for preventative maintenance. On OBD-II vehicles (1996 and up), this replacement interval is 100K miles. About half of the nation’s police cars reach this magic number while still in service.
Two critical engine components have exotic tips that deteriorate with age and greatly affect the combustion cycle: platinum-tipped spark plugs and platinum-wrapped, zirconium-tipped O2 sensors. Change O2 sensors when you change spark plugs. Don’t let your officers use black tape over the MIL. Don’t tell them to keep driving. Fix the car.


Published in Police Fleet Manager, Jul/Aug 2006

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