General Oxygen Sensor FAQ

The oxygen sensor monitors the engines air to fuel ratio by measuring the amount of free oxygen in the exhaust. It reports this information to the engine control unit or ECU. Approximately every 10 milliseconds, the ECU uses this information to make corrections to the air to fuel mixture for maximum efficiency.

An oxygen sensor is a solid electrolyte galvanic cell. Electrolyte is zirconium oxide, which is the same material synthetic diamonds are made from. This material is stabilized with a rare element called Yttria. The cell (thimble or flat plate) is then coated with pure platinum, which acts as a conductor/electrode (like battery plates).
The sensor works as a result of the varying quantities of oxygen in the exhaust verses the amount in the atmosphere. Voltage is produced by the difference in the two amounts. If the amount of oxygen in the exhaust is closer to the amount in the air, the engine is lean and the voltage is low (under 250mv). If the engine is rich, the voltage is high (about 950mv).

No. The base sensor design and elements vary greatly. Below is a list of the most widely used base sensors.

  • Single wire unheated standard zirconia sensors.
  • Two wire unheated standard zirconia sensors.
  • Three wire unheated standard zirconia sensors.
  • Three wire heated titania sensors.
  • Four wire heated standard zirconia thimble type isolated ground sensors. Within this sensor group there are multiple heater types.
  • Four wire heated standard zirconia thimble type case ground sensors.
  • Within this sensor group there are multiple heater types.
  • Four wire heated standard zirconia planar type isolated ground sensors.
  • Within this sensor group there are multiple heater types.
  • Four wire heated zirconia thimble type air fuel ratio sensors.
  • Four wire heated zirconia planar type air fuel ratio sensors.
  • UEGO five wire wide air fuel ratio sensors.

A universal sensor is a base sensor, which does not include the direct fit connector. Splicing is necessary for installation. Universals were widely used in the early years of vehicles equipped with oxygen sensors.

The possibility exists that the user could select the wrong base sensor by assuming that all sensors with the same wire count are equal. All Oxygen Sensors are not created equally. Each type is matched to the make, model, and sub-model application and cannot be mixed. It is impractical to offer a universal sensor for many applications due to heater types, ground types, and other characteristics. Improper selection of the universal sensor could result in serious damage to the engine management system, including failure of the engine control unit (ECU) and/or the catalytic converter.

Natural aging, shock from accidents, antifreeze poisoning, excessive oil consumption or leakage, silicone poisoning due to incorrect use of silicone gasket sealers, etc.

Typically the colors depict the connections inside the sensor; however, various manufacturers choose different colors. For example, some manufacturers use white wires for the heater and others use black. For this and many other reasons, it is best to use a direct fit connector to avoid confusion.

Technical Oxygen Sensor FAQ

To more easily remove an oxygen sensor, soak the sensor thread area with a powerful penetrating lubricant. Starting and revving the engine should further aid in loosening the sensor by heating up the bung. If you are using an open end wrench, try an O2 socket.  If this fails, try a long ratchet or breaker bar in conjunction with your socket to generate more torque. If you are still unsuccessful, heat the bung with a torch until cherry red and remove the sensor. After the sensor is removed, be sure to use a thread cleaner to clean up the bung threads.

In some cases, the threads will need to be repaired. This can be done with a thread repair kit (Walker Part # 88-832). Do not use an impact wrench to remove an O2 sensor, as you will most likely strip the threads in the bung.

Our Find Your Part lookup under the E-Catalog Tab can give you specific sensor data for your vehicle. Modern cars can have up to 6 or more O2 sensors.

In some cases, your sensors will be easily visible on your exhaust. A more detailed description and diagrams of common O2 sensor locations can be found on the Oxygen Sensor page.

The best way to determine the exact locations for your vehicle’s sensors is to consult a maintenance manual such as those put out by Haynes or Chilton.

The role of your downstream sensors is to monitor the output and health of your catalytic converter. Removing them will take away this function and produce a CEL (check engine light) or MIL (malfunction indicator light) on your vehicle.
Not necessarily. The oxygen sensor simply reports the data that it gathers. For example, if you are getting a lean mixture code, you may have a vacuum leak or a faulty fuel injector. Replacing the oxygen sensor will not fix this problem. You will just get the same code again. You can find more information about diagnostic codes for your oxygen sensors on the Oxygen Sensor page.

It is best to replace your sensors in pairs. For example, if you replace the downstream left sensor, you should also replace the downstream right. However, on most vehicles produced since 1996, replacing one sensor (especially the front engine monitoring sensor) will cause the ECU to set a code for the other sensors.

This is because the new sensor switching activity is much faster than that of the older aged sensors. It is probable that on most vehicles, the code will be set within 30-60 days AFTER the first sensor replacement.

An orange hue indicates lead poisoning, black indicates carbon buildup, and white can be a sign of silicone poisoning or antifreeze contamination. Repairs should be made at the source of the trouble, and the sensors need to be replaced. A more complete list of oxygen sensor symptoms and their causes can be found on the Oxygen Sensor page.

Heated oxygen sensors should be checked or replaced every 60,000 miles, while unheated or one wire oxygen sensors should be checked or replaced every 30,000 miles. See our oxygen sensor page.

You can test the O2 sensor on a vehicle by first identifying the signal wire on the sensor. Secondly, by using a voltmeter with the scale set to 1 volt, the voltage will fluctuate between 200 and 800 millivolts or .2 to .8 volts on your meter. If the sensor’s reading is stalled in position, or switches abnormally high or low, your sensor has failed. If your results are inconclusive, it’s best to have your vehicle checked at a professional shop.
Note: This test will not work on Air Fuel or Wide Band sensors.

A second method is to connect some of the various testers available on the market directly to the oxygen sensor. This method is not as accurate, but can detect some of the sensor failures.

A California emissions O2 sensor is meant for vehicles that are designed to meet California emission regulations. Such vehicles should have a sticker under the hood or on the driver’s door jamb that identify them.

Bank 1, containing cylinder # 1, is always the most forward cylinder on the block. Finding Bank 1 is not difficult. The front of the engine will have the accessory pulleys and drive belts, regardless of orientation in the engine compartment. There will be a visible difference in the cylinder head location.

Refer to the diagrams on the Oxygen Sensor page. Sensor 1 will be the pre-catalytic position and Sensor 2 would typically be the post-catalytic position. In some instances, Sensor 2 can be pre-catalytic and thus making Sensor 3 post-catalytic.

Left and right sensor positions are found in reference to the rear of the engine (the side opposite of the belts). Upstream (pre-cat) and downstream (post-cat) are found in reference to the catalytic converter.

See more detailed diagrams and description on our on the Oxygen Sensor page.

Ignition Coil FAQ

The coil is a compact, electrical transformer that boosts the battery’s 12 volts to as high as 40,000 volts.

To understand how ignition coils operate requires that you understand some basic principles of electronics.
You probably didn’t think that you were going to get a short physics lesson, but here goes:

Faraday’s Law and Auto Ignition
How do you obtain 40,000 volts across a spark plug in an automobile when you have only 12 volts DC to start with? The essential task of firing the spark plugs to ignite a gasoline-air mixture is carried out by a process which employs Faraday’s law.

The primary winding of the ignition coil is wound with a small number of turns and has a small resistance. Applying the battery to this coil causes a sizable DC current to flow. The secondary coil has a much larger number of turns and therefore acts as a step-up transformer. But instead of operating on AC voltages, this coil is designed to produce a large voltage spike when the current in the primary coil is interrupted. Since the induced secondary voltage is proportional to the rate of change of the magnetic field through it, opening a switch quickly in the primary circuit to drop the current to zero will generate a large voltage in the secondary coil according to Faraday’s Law. The large voltage causes a spark across the gap of the spark plug to ignite the fuel mixture. For many years, this interruption of the primary current was accomplished by mechanically opening a contact called the “points” in a synchronized sequence to send high voltage pulses through a rotary switch called the “distributer” to the spark plugs. One of the drawbacks of this process was that the interruption of current in the primary coil generated an inductive back-voltage in that coil which tended to cause sparking across the points. The system was improved by placing a sizable capacitor across the contacts so that the voltage surge tended to charge the capacitor rather than cause destructive sparking across the contacts. Using the old name for capacitors, this particular capacitor was called the “condenser.”

More modern ignition systems use a transistor switch instead of the points to interrupt the primary current.

The transistor switches are contained in a solid-state Ignition Control Module. Modern coil designs produce voltage pulses up in the neighborhood of 40,000 volts from the interruption of the 12 volt power supplied by the battery.
Some modern engines have multiple ignition coils mounted directly on the spark plugs. Instead of single voltage pulses, they may under some engine conditions produce three voltage pulses. The coil arrangement shown is on a Dodge engine.

The potting materials have changed dramatically since the ignition coils inception; originally ignition coils were paper insulated wiring coils mounted in wood or steel boxes. Later, the ignition coil was built into the familiar conical shape and filled and formed with a phenolic plastic potting material to protect it from vibration and heat.
The potting materials have changed from phenolic plastic to epoxys, urethane polymers, and most recently to DuPont Rynite® and Thermx®. The materials may look like the same black plastic, but they have evolved dramatically.

Some early ignition coils were oil cooled (the windings were submerged in oil), which could leak and cause overheating and coil failure, but all modern ignition coils are now dry due to improvements in core materials, coil winding materials, and potting materials that can withstand higher under hood temperatures, vibration, and higher secondary voltage.

Primary and secondary coil windings use an insulated copper or copper coated aluminum wire. Originally the windings were insulated with paper or cloth. Most modern coil windings are now insulated with polyurethane or polyamide enamel composites.

All ignition coils have a magnetic core made of ferromagnetic metal such as iron, or ferromagnetic compounds such as ferrites to concentrate the strength and increase the effect of magnetic fields produced by the electric current.

The presence of the magnetic core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core.

Some modern COP (Coil on Plug) systems (2102 & UP) monitor Ionization current to determine a number of factors such as misfire detection, knock detection, fuel compensation, and proper timing advance for the next firing sequence, etc. These COP systems are capable of inducing over 50,000 volts if required (dictated by the required ionization current).

Delphi’s Ionization Current Sensing Ignition Subsystem (Ion Sense) is a technology based on the principle that electrical current flow in an ionized gas (e.g. during combustion) is proportional to the flame electrical conductivity. By placing a direct current bias on the spark plug electrodes, the conductivity can be measured.

With Ion Sense technology, the conventional spark plug acts as an intrusive sensor in the cylinder to obtain information about each combustion event with minimal influence due to environmental conditions such as vibration, mechanical noise, and temperature. Optimized individual cylinder knock control helps increase engine efficiency and reduce fuel consumption. Through Ion Sense technology, misfire detection is OBD II capable and provides very high reliability and robustness compared to many other detection methods.

Advanced features of Ion Sense Subsystems, such as compensation of combustion due to fuel variation, are also available to help reduce cold-start HC tailpipe emissions.

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