Recently I read a good article in the January issue of Strar Tune December 2005 volumn 5 number 5. The article goes on to decribe early 02 sensors and the new wide band planar sensors . the difference is that instead of ceramic thimble, heated planar sensors utilize a flat ceramic zirconia element, less than two millimeters thick, projecting into the exhaust stream. The electodes conductive layer of ceramic , and heater are laminated into a unified layer strip. This strip is smaller, lighter and more resistant to contamination than the thimble design. The planar sensor intergrated heater element also requires less electrical power to reach operating temperature. that is the real advantage when you consider the number of amps necessary to bring a heated thimple type up to the 600 deg. F. required.

Some discussion was breifly discussed but not finalized as how to properly test
o2 senors. Mercedes-Benz does not recommend any avasive test that can comprimise the ground wire by probing. what other ways are safe to test these senors?
Bob Geco

Bob, what kind of car??

Many different types of sensors require different testing methods.

The easiest cars(MB) were from 1980 to 85. Single wire no heater.

Then the 3 wire sensors from 1986 were almost the same type test..

From there it can get complex

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Documentation for checking most current model Oxygen sensors...... I'm not the author.....

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.
WARNING
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.

__________________
Russell McMahon
Tulsa, OK
2001 CLK 320
1992 400se
1993 400e
2000 320ML

Russell:
thank you for the testing proceeder. I have a good DOM meter and a snap-on scaner. I also read this star tunes issue December 2005 on the new Planar 02 sensors . I do not know if my sensor is the newer type or not . I will try you advice and hopethat I can learn something.

thank you
Bob Geco