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O2 Sensor Failures O2 SENSOR FAILURES

O2 sensor performance can deteriorate as a result of age, contamination or damage. This causes a decrease in reaction time to changes in the air/fuel ratio as well as a lower voltage output. Eventually the point may be reached where the sensor fails to switch at all.

Symptoms of a failed O2 sensor include:
* Failed emissions test (high CO typically, but sometimes high HC too)
* Damaged catalytic converter (from an over rich fuel mixture)
* Poor fuel mileage (caused by over rich fuel mixture)
* Fouled spark plugs (caused by over rich fuel mixture)
* Runs rough
* Sluggish performance If the average voltage from the O2 sensor is running high (more than 0.50v), it indicates a rich condition, possibly due to a bad MAP sensor or leaky injector. If the average voltage reading is running low (less than 0.45v), the mixture is running lean possibly due to a vacuum leak, air leak at the exhaust manifold, dirty fuel injectors, a weak fuel pump or because the O2 sensor itself is bad.

If the O2 sensor continually reads high (rich), it will cause the engine computer to lean out the fuel mixture in an attempt to compensate for the rich reading. This can cause lean misfire, hesitation, stumbling, poor idle and elevated hydrocarbon emissions (from misfiring). If the O2 sensor continually reads low (lean), it will cause the engine computer to richen the fuel mixture. Injector dwell or pulse width will increase causing fuel consumption and carbon monoxide emissions to go up. A continually rich fuel mixture can also cause the catalytic converter to overheat be damaged.

If the O2 sensor's output is sluggish and does not change (low cross counts & long transition times), the engine computer will not be able to maintain a properly balanced fuel mixture. The engine may run too rich or too lean, depending on the operating conditions. This, in turn, may cause drivability problems such as misfiring, surging, poor idle, and elevated emissions.

If a heated O2 sensor has a faulty heating circuit or element, the sensor can cool off at idle causing the system to go into open loop. This usually results in a fixed, rich fuel mixture that will increase emissions. Sometimes an apparent O2 sensor problem is not really a bad sensor. An air leak in the intake or exhaust manifold or even a fouled spark plug or a burned exhaust valve that allows unburned air to pass to the exhaust can cause the O2 sensor to give a false lean indication. The sensor reacts only to the presence or absence of oxygen in the exhaust. It has no way of knowing where the extra oxygen came from. So keep that in mind when troubleshooting O2 sensor problems.

Single wire O2 sensors are grounded through the exhaust manifold. If rust and corrosion creates resistance, it may affect the sensor's output. To rule out a bad ground, use a digital volt meter to check for a voltage drop between the sensor shell and the engine block. More than 0.1v can cause a problem.

Everybody knows that spark plugs have to be replaced periodically to maintain peak engine performance, but many people don't realize the same goes for oxygen sensors. As long an O2 sensor is working properly, there's no reason to replace it. But after many miles of being constantly bathed in hot exhaust gas, a buildup of deposits on the sensor tip can make it sluggish. If there's enough gunk on the sensor tip, the sensor may produce little or no voltage at all. This produces a false "lean" signal that makes the computer think the engine needs more fuel, which it doesn't but gets anyway. This creates a rich fuel condition that kills fuel economy and sends carbon monoxide emissions soaring. The engine may also experience additional drivability problems such as surging or hesitation.

The same kind of thing can happen if the O2 sensor has been contaminated by deposits from sources other than normal combustion. Possible causes of contamination include harmful fuel additives, lead, silicone (from antifreeze leaking into a cylinder or from using the wrong type of RTV sealer), and phosphorus from burning oil.

Oil contamination will leave dark brown deposits on the sensor tip, while antifreeze contamination will leave light colored grainy deposits. Black carbon deposits on the sensor tip may indicate a very rich fuel condition. White or reddish deposits indicate fuel contamination. Older vented O2 sensors can also be contaminated from the outside by spilled oil, overspray of rustproofing or other chemicals (degreasers, lubricants, etc.). Dirt or water that enters the vent hole on vented single-wire sensors can also cause problems. Physical damage is also possible due to the sensor's location on the exhaust manifold. If you suspect an O2 sensor problem, the first thing you should do is check for any codes that would implicate the sensor circuit. A code by itself doesn't necessarily mean the O2 sensor is bad, however. It might be a wiring problem or something else. So always check the operation of the sensor to see if is is functioning properly before you replace it. If there are no codes, that doesn't necessarily mean the O2 sensor is okay. In many instances, a sluggish O2 sensor may not be bad enough to set a fault code but will still cause an emissions or drivability problem.

According to one California study, 70% of all fuel-injected vehicles that failed the state's emissions test failed because they had bad O2 sensors. Yet few of these vehicles had check engine lights or fault codes that indicated a faulty O2 sensor.

Diagnostic procedures vary by vehicle manufacturer, but the following will work on most applications:

1. First, verify basic engine health. Make sure compression, ignition, fuel delivery and induction system are operating normally.
2. Run the engine to operating temperature and turn the engine off.
3. Disconnect the O2 sensor lead and attach the red (positive) voltmeter test lead to the sensor signal wire, and the black (negative) test lead to to ground.
4. Restart the engine and run it at 2,500 rpm.
5. Artificially enrich the fuel mixture by directing propane into the intake manifold until the engine speed drops 200 rpm. The voltmeter should rapidly show an O2 sensor output voltage of 900 mv (0.9 volts) indicating a rich fuel mixture. No change or a low voltage output would indicate a faulty O2 sensor.
6. Next, stop the propane and create a small vacuum leak to lean the fuel mixture. The voltmeter should now show a drop in the O2 sensor's output to around 200 mv (0.2 volts).
7. To check the sensor's dynamic response, turn the engine off and reconnect the sensor lead. Then start the engine and run at 1,500 rpm. The sensor's average output should fluctuate around 450 to 500 mv (0.45 to 0.50 volts).

The O2 sensor's output signal can also be observed on an oscilloscope. Leave the sensor wires connected and backprobe the connector with the scope leads. Connect the scope lead to the sensor signal wire, and ground to ground. A "good" waveform should show an output voltage that changes back and forth from about 800 mv (0.8 volts) to 200 mv (0.2 volts). The signal should respond to changed in the fuel mixture within 300 milliseconds or less. GM says a good sensor should be able to switch from rich to lean in less than 125 milliseconds, and from lean to rich in less than 100 milliseconds.

CAUTION! Never use an ohmmeter on a zirconium O2 sensor in an attempt to check the sensor because doing so can damage it. And never jump or ground the sensor's leads.

One or two wire zirconium O2 sensors can also be bench tested by heating the tip with a propane torch while monitoring the sensor's voltage output with a digital voltmeter. Connect the positive voltmeter lead to the wire coming out of the O2 sensor and the negative voltmeter lead to the sensor's outer shell. Then heat the tip of the sensor with the propane torch. The tip should be hot enough to turn cherry red, and the flame must enter the opening into the sensor tip. If you get a voltmeter reading above 600 millivolts (0.6 volts), and the reading quickly changes as you move the flame back and forth over the tip, the sensor is okay. A low reading or one that is slow to change means the sensor needs to be replaced.

Replacement sensors must be the same basic type as the original (heated or unheated) and have the same performance characteristics and heater wattage requirements.

WARNING: Installing the wrong O2 sensor could affect engine performance and possibly damage the heater control circuit in the engine computer. Most replacement sensors have a conductive anti-seize coating on the threads to make future replacement easier. If the threads are not coated, apply some anti-seize to the threads. Be careful not to get any anti-seize on the sensor shell or vented tube.

One way to assure maximum engine performance, the lowest emissions and best possible fuel economy is to replace aging O2 sensors before they get too old and start causing rouble.

Bosch recommends replacing unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s applications every 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles.

On 1996 and newer OBD II equipped vehicles, the recommended replacement interval is 100,000 miles.

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