Optimize Your Engine with Quality Oxygen (O2) Sensors

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Purpose

The oxygen (O2) sensor is the principal sensor in the fuel mixture feedback control loop. The oxygen sensor monitors the vehicle’s exhaust and sends a voltage signal, which represents the amount of oxygen in the exhaust, to the vehicle’s computer system.

The oxygen sensor responds to unburned oxygen in the vehicle's exhaust. When the fuel mixture is "rich" (indicated by too little oxygen in the exhaust), the sensor generates a high voltage signal. When the fuel mixture is lean (indicated by excessive oxygen in the exhaust), the sensor generates a low voltage signal.

The computer increases fuel delivery for “lean” exhaust conditions (too much oxygen) and decreases fuel delivery for “rich” exhaust conditions (too little oxygen).A vehicle may be equipped with one or more oxygen sensors.

Location

Oxygen sensors are threaded into exhaust manifold.

Operation

The oxygen sensor consists essentially of a zirconium dioxide ceramic sensor element with a platinum coating on either end that serve as electrodes. One electrode is exposed to oxygen in the vehicle's exhaust. The other electrode is exposed to oxygen in the atmosphere. The ceramic sensor element becomes electrically conductive at high temperature (typically 600 degrees F minimum).

When the oxygen sensor is heated to the correct temperature and different concentrations of oxygen are present on either side of the sensor element, the sensor generates a voltage across the electrodes. This generates a output voltage that varies with the engine's air/fuel mixture.

When the fuel mixture is "rich", most of the oxygen is consumed during combustion, and little unburned oxygen remains in the vehicle's exhaust. The sensor's output is high when the fuel mixture is "rich" (low oxygen content in the vehicle's exhaust), and low when the mixture is "lean" (high oxygen content in the vehicle's exhaust).

The vehicle's computer uses the output fro the oxygen sensor to rebalance the fuel mixture for the lowest possible emissions. When the sensor indicates a "lean" condition, the computer increases the on-time of the fuel injectors to "enrich" the air/fuel mixture. When the sensor indicates a "rich" condition, the computer reduces the on-time of the injectors to "lean out" the air/fuel mixture. This causes a back-and-forth between "rich" and "lean" as the computer tries to maintain the optimum air/fuel mixture for the current driving conditions.

There are five basic types of oxygen sensors. Each type is described in the following paragraphs.

Unheated Thimble-Type Oxygen Sensors

The unheated thimble-type oxygen sensor is the oldest design. The sensing element is shaped like a long thimble, and is housed inside a metal tube that extends into the exhaust manifold. Holes in the tip of the tube allow the exhaust gases to contact the sensing element. A vent hole in the sensor shell facilitates contact with the outside atmosphere. The sensor produces a voltage signal that switches back and forth as the air/fuel mixture changes.

Unheated oxygen sensors rely on heat from the exhaust to reach normal operating temperature. As a result, it may take up to several minutes after a "cold" start for the sensor to generate a signal. Unheated sensors may also "cool off" at idle, causing the vehicle's computer to return to "open loop" operation.

Heated Thimble-Type Oxygen Sensors

The heated thimble-type oxygen sensor uses the same sensing element as the unheated sensors, but incorporates a heating element that brings the sensor up to operating temperature within 30 to 60 seconds. The heater allows the engine to enter closed loop operation more quickly, which reduces cold start emissions. It also prevents the sensor from "cooling off" at idle.

Heated Titania-Type Oxygen Sensors

Titania-type oxygen sensors use a different sensing element that produces a different signal than the thimble-type sensors; the sensor's resistance varies with changes in the oxygen content in the vehicle's exhaust. The resistance ranges from a low resistance when the air/fuel mixture is "rich" to a high resistance when the air/fuel mixture is "lean." The vehicle's computer provides a reference voltage to the sensor, and then monitors the change in the sensor's voltage output signal as the sensor's resistance changes. A "lean" mixture produces a low voltage signal, while a "rich" mixture produces a high voltage signal. These types of sensors are used in a limited number of applications.

Heated Planar-Type Oxygen Sensors

The heated planar-type oxygen sensor uses a flat ceramic zirconia element instead of a thimble-type. The ceramic sensor, electrodes, insulation and heater are all laminated into a single strip, minimizing the destructive effects of contaminants in the vehicle's exhaust. The sensor also incorporates an improved heating element that allows the sensor to reach normal operating temperature in approximately 10 seconds. Operation of the sensor is similar to the thimble-type sensor.

Heated Wide-Band Type Oxygen Sensors

The wide-band type oxygen sensor uses a "dual sensing element" that allows the sensor to directly and precisely measure air/fuel ratios from very "rich" to very "lean." The sensor produces a low voltage signal when the air/fuel mixture is "lean", and a high voltage signal when the air/fuel mixture is "rich." But instead of switching abruptly as is the case with the older style sensors, it produces a gradual change in the voltage that increases or decreases in proportion to the "richness" or "leanness" of the air/fuel mixture. This allows the vehicle's computer to add or subtract fuel as necessary to maintain the optimum air/fuel mixture.

Drivability Symptoms

The following symptoms may indicate an aged, damaged or defective oxygen sensor, or trouble in a related system:

• Stalling, stumbling or rough idle
  
• Hesitation
  
• Degraded fuel economy
  
• Air/fuel ratio problems (fuel mixture is excessively "rich" or excessively "lean")
  
• Poor engine performance
  
• Fouled spark plugs
  
• Damaged catalytic converter
  
• Inability to pass an emissions test (due to excessive carbon monoxide or hydrocarbons in the vehicle's exhaust)
  

Testing

The procedures for testing oxygen sensors vary depending on vehicle make and model, and on the specific type of sensor. Refer to the vehicle's service manual for test equipment requirements and test procedures.

Troubleshooting

If the oxygen sensor's response is slow, the vehicle's computer will not be able to maintain the correct air/fuel mixture.

If a heated O2 sensor has a faulty heating circuit or element, the sensor may "cool off" when idling, causing the system to go into open loop.

Trouble in related systems may also affect oxygen sensor operations. If a defective oxygen sensor is suspected, be sure to check the following areas:

• If the oxygen sensor output voltage is continually high (indicating a "rich" air/fuel mixture), a bad MAP sensor or leaky fuel injector may be indicated.
  
• If the oxygen sensor output voltage reading is continually low (indicating a "lean" air/fuel mixture), a vacuum leak may be present.
  
• An air leak in the intake or exhaust manifold, or a fouled spark plug, may cause the O2 sensor to indicate a false "lean" condition.
  

Repair/Replacement

The BARO sensor is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible.

The oxygen sensor's ability to rapidly respond to changes in oxygen content in the vehicle's exhaust is critical to operation of the vehicle's fuel mixture feedback control loop. The oxygen sensor's responsiveness can be affected by both age and exposure to contaminants in the exhaust, causing the sensor's voltage output to decline, giving a lower than normal reading. As a result, the vehicle's computer will respond as if the fuel mixture is "leaner" than it actually is, resulting in an overly "rich" air/fuel mixture. Regular replacement of the oxygen sensor can prevent this problem.

• Unheated one- or two-wire oxygen sensors (used on vehicles from 1976 through the early 1990s) applications should be replaced every 30,000 to 50,000 miles.
  
• Heated three- and four-wire oxygen sensors (used on vehicles from the mid-1980s through mid-1990s) applications should be replaced every 60,000 miles.
  
• Oxygen sensors used on OBDII-compliant vehicles should be replaced every 100,000 miles.
  

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