I have been collecting O2 sensor info over the years. Half of the following is in my own words so feel free to correct anything you see wrong.
The O2 sensor is there primarily because of emissions! A second reason is to fine tune fuel management. And thirdly they indicate the state of the catalyst.
It was developed to ensure that the correct amount of oxygen is available for a reaction to take place in the CAT which needs O2 to convert other nasty gases to form less nasty gases. (Example carbon monoxide + oxygen = CO2.
The sensors upstream (before the Cat) are important for delivering the correct amount of oxygen and for fine tuning the air-to fuel ratio. They help determine the duration of time that the injectors stay open. The oxygen sensor measures the oxygen content from the exhaust stream and from this it can figure out if the fuel mixture is rich or lean
The downstream O2 sensors are for measuring the catalyst shelf-life. This will tell if the Cat is spent and needs replacement. This sensor has no effect on the engine performance if it the vehicle is OBDI (up to 1995).
A rich mixture consumes most or all of the oxygen so the voltage generated by the O2 sensor will be high (0.6 to 1.0 volts). A leans mixture will have more available O2 left over. In this case the sensor will produce a voltage from 0.1to 0.4v. The perfect average voltage is around 0.45 v for complete combustion.
The sensor will only generate an accurate signal when it is at a minimum of 400 C, so it is ignored until the engine control goes into Closed Loop operation (an engine at operating temperature). The O2 sensor also has its own built in heater (2amp load) to get it to temp quickly and so that it is hotter then the exhaust stream. This cuts down the emissions as well.
Both O2 sensor measures how much O2 is present in the exhaust stream. The computer does a simple calculation (upstream O2 value – downstream O2 value). Obviously, the upstream valve is always a higher valve. Since a chemical reaction takes place, there is less O2 exiting the Cat. So the math as I see it:
If (upstream O2 value – downstream O2 value) = a negative number, than one of the sensors is bad.
If (upstream O2 value – downstream O2 value) = 0, then the catalyst is spent and needs to be replaced.
If (upstream O2 value – downstream O2 value) = a positive number, then the catalyst is converting oxygen and other constituents to form less toxic gases.
Testing the Heater
You can easily test the heater circuit with the two white wires. With the engine cold and off unplug the O2 sensor connector. Check for resistance on the sensor side. An Open circuit or mega-ohms means a bad heater circuit. Throw the O2 sensor in the trash. . I measured 4.5 ohms at 45 degrees F outside. This will always store a code. The heater circuit should draw about 2 amps when hot.
Testing the measuring circuit
The output of the measuring circuit is more difficult. With a hot engine one can back-probe the connector. The voltage output is a Sine-wave. So if you connect a voltmeter across the correct pins (gray & black wires), then the meter will constantly bounce back & forth between 0.1 & 0.9 volts. A good sensor will bounce between a much narrower range like (0.25 to 0.75). The important thing is that the average of the swing equals 0.45 volts. I have not tried to measure this but I think it will be too difficult to measure on a voltmeter unless it has an averaging function. (only the really cool & expensive ones have this)
While measuring this circuit, you can pull off a vacuum hose and check to see if the voltage drops (a lean mixture)
A bad upstream sensor can cause a decrease in mileage, higher CO and/or HC emission levels
Bank 1 sensor 1= the side of the engine which has cylinder number 1 on it and this is upstream of the Cat. This is the driver’s side on Jeeps.
Bank 1 sensor 2 = driver’s side downstream sensor or after the Cat.
Bank 2 sensor 1 = passenger side upstream sensor
Bank 2 sensor 2 = passenger side downstream sensor
Orange/with black stripe goes to white wire on sensor (heater 12 volts DC)
Black/with light blue stripe goes to the other white wire on sensor (heater ground)
Black with orange stripe goes to the gray wire on sensor (0.1 to 0.9 volts AC signal from sensor)
Black with tan stripe goes to black wire on sensor (sensor ground)
Many things can cause the sensor to read incorrectly: the built-in heater circuit not working, supplemental gas additives, vacuum leak, and ERG not working can all cause the O2 sensor to appear to be bad.
Unfortunately, the best way to determine if the sensors are bad is to have a proper scan performed. That way the outputs are plotted and it can be determined if a sensor is lazy or not functioning properly
Many people have reported problems with aftermarket O2 sensors! I would go to your local Dodge or Jeep dealer and give them your VIN number. That way you hand you a sensor that will work correctly with your Jeep; or email Nick at Kolak@aol.com and tell him that you got his name from this forum for a discount.