Oxygen Sensor


Oxygen sensors produce a voltage based on the amount of oxygen in the exhaust. Large amounts of oxygen result from lean mixtures and result in low voltage output from the O2 sensor. Rich mixtures release lower amounts of oxygen into the exhaust, therefore, the O2 sensor voltage is high. The engine must be at normal operating temperature before the oxygen sensor is tested. Always refer to the specifications supplied by the manufacturer.

Before testing an O2 sensor, refer to the correct wiring diagram to identify the terminals at the sensor. Most late-model engines use heated oxygen sensors (HO2S). These sensors have an internal heater that helps to stabilize the output signals. Most heated oxygen sensors have four wires connected to them. Two are for the heater and the other two are for the sensor.

Use a wiring diagram to identify the terminals on a heated oxygen sensor. Courtesy of American Honda Motor Co., Inc.
An oxygen sensor must be tested with a digital voltmeter. If an analog meter is used for this purpose, the sensor may be damaged.

Testing With A Digital Mulitmeter

  • Connect the voltmeter between the O2 sensor wire and ground.
  • Backprobe the connector near the O2 sensor to connect the voltmeter to the sensor signal wire. If possible, avoid probing through the insulation to connect a meter to the wire.
  • With the engine idling, the sensor voltage should be cycling from low voltage to high voltage. The signal from most O2 sensors varies between 0 and 1 volt.
  • If the voltage is continually high, the air/fuel ratio may be rich or the sensor may be contaminated by RTV sealant, antifreeze, or lead from leaded gasoline.
  • When the O2 sensor voltage is continually low, the air/fuel ratio may be lean, the sensor may be defective, or the wire between the sensor and the computer may have a high-resistance problem.
  • If the O2 sensor voltage signal remains in a mid-range position, the computer may be in open loop or the sensor may be defective.
  • The sensor can also be tested after it is removed from the exhaust manifold.
  • Connect the voltmeter between the sensor wire and the case of the sensor.
  • Using a propane torch, heat the sensor element. The propane flame keeps the oxygen in the air away from the sensor element, causing the sensor to produce voltage.
  • While the sensor element is in the flame, the voltage should be nearly 1 volt.
  • The voltage should drop to zero immediately when the flame is removed from the sensor.
  • If the sensor does not produce the specified voltage or if the sensor does not quickly respond to the change, it should be replaced.
  • If a defect in the O2 sensor signal wire is suspected, backprobe the sensor signal wire at the computer and connect a digital voltmeter from the signal wire to ground with the engine idling.
  • The difference between the voltage readings at the sensor and at the computer should not exceed the vehicle manufacturer's specifications. A typical specification for voltage drop across the average sensor wire is 0.02 volt.
  • Now check the sensor's ground.
  • With the engine idling, connect the voltmeter from the sensor case to the sensor ground wire on the computer. Typically, the maximum allowable voltage drop across the sensor ground circuit is 0.02 volt.
  • Always use the vehicle manufacturer's specifications. If the voltage drop across the sensor ground exceeds specifications, repair the ground wire or the sensor ground in the exhaust manifold.
  • Most late-model engines are fitted with heated O2 sensors.
  • If the O2 sensor heater is not working, the sensor warm-up time is extended and the computer stays in open loop longer. In this mode, the computer supplies a richer air/fuel ratio. As a result, the engine's emissions are high and its fuel economy is reduced.
  • To test the heater circuit, disconnect the O2 sensor connector and connect a voltmeter between the heater voltage supply wire and ground.
  • With the ignition switch on, 12 volts should be supplied on this wire. If the voltage is less than 12 volts, repair the fuse in this voltage supply wire or the wire itself.
  • With the O2 sensor wire disconnected, connect an ohmmeter across the heater terminals in the sensor connector.
  • If the heater does not have the specified resistance, replace the sensor.

Testing With A Scanner

The output from an O2 sensor should constantly cycle between high and low voltages as the engine is running in closed loop. This cycling is the result of the computer constantly correcting the air/fuel ratio in response to the feedback from the O2 sensor.

When the O2 sensor reads lean, the computer will enrich the mixture. When the O2 sensor reads rich, the computer will lean the mixture. When the computer does this, it is in control of the air/fuel mixture. Many things can occur to take that control away from the computer. One of them is a faulty O2 sensor.

  • The activity of the sensor can be monitored on a scanner.
  • Watch the scanner while the engine is running.
  • The O2 voltage should move to nearly 1 volt then drop back to close to zero volt.
  • Immediately after it drops, the voltage signal should move back up. This immediate cycling is an important function of an O2 sensor.
  • If the response is slow, the sensor is lazy and should be replaced. With the engine at about 2,500 rpm, the O2 sensor should cycle from high to low ten to forty times in 10 seconds.
  • The voltage readings shown on the scanner are also an indicator of how well the sensor works.
  • When testing the O2 sensor, make sure the sensor is heated and the system is in closed loop.

Testing With A Lab Scope

Correct way to connect a lab scope to an oxygen sensor.
  • A faulty O2 sensor can cause many different types of problems.
  • For example, it can cause excessively high HC and CO emissions and all sorts of driveability problems.
  • Most computer systems monitor the activity of the O2 sensor and store a code when the sensor's output is not within the desired range.
  • Again, the normal range is between 0 and 1 volt and the sensor should constantly toggle from close to 0.2 to 0.8 volt, then back to 0.2.
A good O2 sensor trace. Courtesy of Progressive Diagnostics - WaveFile AutoPro.
  • If the range that the sensor toggles in is within the specifications, the computer will think everything is normal and respond accordingly. This, however, does not mean the sensor is working properly.
  • The voltage signal from an O2 sensor should have two to three cross counts with the engine without a load at 2,000 rpm. A cross count of an O2 sensor is the number of times the waveform moves from a high voltage and back to that voltage.
O2 sensor signal cross counts. Courtesy of OTC Tool and Equipment, Division of SPX Corporation.
  • If the sensor's voltage toggles between zero volt and 500 millivolts, it is toggling within its normal range but it is not operating normally. It biased low or lean. As a result, the computer will be constantly adding fuel to try to reach the upper limit of the sensor. Something is causing the sensor to be biased lean. If the toggling only occurs at the higher limits of the voltage range, the sensor is biased rich. In either case, the computer does not have true control of the air/fuel mixture because of the faulty O2 signals.
  • The O2 can be biased rich or lean, not work at all, or work too slowly to ensure good emissions and fuel economy.
  • To test the O2 sensor for all of these concerns, use a lab scope. Begin by allowing the engine and O2 sensor to warm up.
  • Insert the hose of a propane enrichment tool into the power brake booster vacuum hose or simply install it into the nozzle of the air cleaner assembly.
  • This will drive the mixture rich. Most good O2 sensors will produce almost 1 volt when driven full rich. The typical specification is at least 800 millivolts.
  • Connect the lab scope to the sensor and a good ground.
  • Set the scope to display the trace at 200 millivolts per division and 500 milliseconds per division.
  • Inject some propane into the air cleaner assembly.
  • Observe the O2 signal's trace. The O2 sensor should show over 800 millivolts. If the voltage doesn't go high, the O2 sensor is bad and should be replaced.
  • Now, remove the propane bottle and cause a vacuum leak by pulling off an intake vacuum hose.
  • Watch the scope to see how the O2 sensor reacts.
  • It should drop to under 175 millivolts. If it doesn't, replace the sensor.
  • These tests check the O2 sensor, not the system, therefore they are reliable O2 sensor checks.
  • Also keep in mind that on an air pump-equipped car, it's a good idea to disable the air pump before doing this test. Unwanted air may bias the results.
  • Observing the trace of an O2 sensor can also help in the diagnosis of other engine performance problems.
O2 sensor signal caused by an ignition problem. Courtesy of Progressive Diagnostics - WaveFile AutoPro.

The above figure shows how ignition problems affect the signal from the O2 sensor. Keep in mind that during complete combustion, nearly all of the oxygen in the combustion chamber is combined with the fuel. This means there will be little O2 in the exhaust of a very efficient engine. As combustion becomes more incomplete, the levels of oxygen increase. Ignition problems cause incomplete combustion and there is much oxygen in the exhaust. This is also true of lean mixtures, over-advanced ignition timing, or anything else that causes incomplete combustion.

When the mixture is rich, combustion has a better chance of being complete. Therefore, the oxygen levels in the exhaust decrease. The O2 sensor output will respond to the low oxygen with a high voltage signal.

O2 sensor signal caused by defective fuel injector. Reproduced with permission from Fluke Corporation.

Remember that the PCM will always try to do the opposite of what it receives from the O2 sensor. When the O2 shows lean, the PCM goes rich, and vice versa. When a lean exhaust signal is not caused by an air/fuel problem, the PCM does not know what the true cause is and will enrich the mixture in response to the signal. This may make the engine run worse than it did.