O2 Sensor on Cat Converter

[chemist]

For the O2 sensor on the catalytic converter--a couple questions.
1. Before I go back under, where on the cat will I find it?
2. How is removed (simple unbolt or what)/

Thanks for all the help so far.

[astroturf]

chemist, You will find 3 o2 sensors under there. 2 on the crossover before the cat, and 1 down line or after the cat. Hope it goes well. Jim

edit: sorry bout that, you'll need a wrench. not sure of size. They are mounted in bungs on exhaust pipes. Jim

[SoCalli V8]

What sensors are you changing? The front two are the most important and should be changed as a pair. I recommend changing them at 120k for the best engine performance and gas milage.

[phr1$k37]

What sensors are you changing? The front two are the most important and should be changed as a pair. I recommend changing them at 120k for the best engine performance and gas milage.

Sorry to hijack the post ... but when you say 120k that is in miles?
What is the best way to tell if O2 needs replacement?

[astroturf]

What sensors are you changing? The front two are the most important and should be changed as a pair. I recommend changing them at 120k for the best engine performance and gas milage.

I'll bite. How do you know when they're bad? Ya, Ya, What phr said. Thanks SoCalli V8, Jim

[SoCalli V8]

As a smog technician, working in the most Smoggy / Anal part of the world (LA) replace them with high quality replacements, because the cheap and cheerful will be junk and inop in a year.

There are strict laws out there now, that will be spreading to your "Non Smog States" It is all in the works, and is gonna be Global. > So Look-out !

[thevalleyboy]

are the bosch ones from Rockauto - decent ones - or cheapies? They seem like a decent price, or are the Delco's or Airtex (never heard of them) the better buy?

[SoCalli V8]

What sensors are you changing? The front two are the most important and should be changed as a pair. I recommend changing them at 120k for the best engine performance and gas milage.

Sorry to hijack the post ... but when you say 120k that is in miles?
What is the best way to tell if O2 needs replacement?

Yes 120k is in miles.

If you have an OBD 2 van it will set a check engine light. However, any OBD1 or 2 oxy sensor can be tested with a multi meter or an oscilloscope hooked into the black signal wire, and the reading should rise and fall between .2 and .9 of a volt at 1800rpm. If it flat-lines or oscillates too slow, it should be replaced.

Bosch are ok, AC Delco are much better and will last longer, but Airtex or any cheaper brand are junk, don`t waste your cash.

[SoCalli V8]

Some Oxygen sensor info for ya.

Understanding oxygen sensors.....


Since the early 1980s, oxygen sensors (O2S) and heated oxygen sensors (HO2S) have played a key role in the efficient operation of electronic fuel injected vehicles. In a modern vehicle, the powertrain control module (PCM) relies on information from the oxygen sensor to achieve optimum air/fuel ratio, good engine performance and control exhaust emissions. Understanding fundamentals of oxygen sensor operation, as well as new changes in technology, can help technicians quickly test and diagnose this increasingly important sensor.

Burning gasoline in the combustion chamber of an engine is a chemical reaction with fairly predictable results. Cylinder misfire, poor engine efficiency and high exhaust emissions can be the end result of too much or too little fuel in the combustion chamber. An oxygen sensor can effectively measure these combustion results. Changes in air-to-fuel ratio affect the amount of oxygen (O2) consumed during the combustion process. The best air/fuel ratio for complete combustion and emissions is a stoichiometric 14:7:1 ratio. A rich (or excessive fuel) air/fuel ratio will consume most of the oxygen during the combustion process, resulting in low exhaust oxygen content. Leaner air/fuel ratios will result in somewhat higher exhaust oxygen content. By monitoring oxygen content of the engine exhaust, the PCM can determine the ideal air/fuel ratio and adjust fuel delivery accordingly.



Oxygen sensors are typically located in the exhaust manifold and/or exhaust system. While earlier fuel injection systems used one or possibly two oxygen sensors, on-board diagnostics II (OBD-II) system emission regulations have warranted the use of multiple oxygen sensors on most vehicles. OBD-II vehicles typically have at least one oxygen sensor located ahead of the catalytic converter (upstream) and an additional sensor located just after the catalyst (downstream).

Using upstream and downstream oxygen sensors enables the PCM to measure efficiency of both engine combustion and catalyst operation.

Vehicles with dual exhaust systems may also have pre- and post-catalyst oxygen sensors for each bank of engine cylinders. The exact placement and number of oxygen sensors varies with engine configuration, vehicle design and manufacturer.

One of the most common types of oxygen sensors is the zirconium dioxide oxygen sensor. The O2 sensing component uses a solid-state electrolyte made up of a zirconic ceramic material that acts like a galvanic battery electrolyte under certain conditions. When the sensing element is cold, the zirconia material behaves similar to an insulator. At elevated temperatures, the zirconia material performs more like a semiconductor, and can generate a characteristic voltage output on the sensor connections.

In construction of the zirconia sensing element, a porous platinum electrode material covers the inner and outer surfaces of the zirconia solid-state electrolyte. The inner surface of the sensing element is exposed to an outside air reference, while hot gases in the exhaust stream surround the sensor's outer portion. Oxygen content of outside air is approximately 21 percent, while exhaust gases have much lower oxygen content - between 1 percent and 3 percent.

Differences in the two oxygen levels, and the electrolytic properties existing between the two platinum electrodes, allow ion transfer to take place and generate a small electrical charge. Oxygen ions are electrically charged particles that flow through the zirconia sensing element when there is a disparity in oxygen levels. The greater the ion flow, the higher the voltage produced. Once the zirconia sensor element reaches an operating temperature of 572 degrees Fahrenheit to 680 degrees Fahrenheit, signal voltage output can range from near zero to 1 volt - depending on the oxygen content of the exhaust gases.

Basically, the zirconium O2 sensor compares the oxygen content of exhaust gases with oxygen from outside air. Voltage produced by the O2 sensor depends on the amount of oxygen in the exhaust. If exhaust oxygen content is low, such as a rich air/fuel ratio, the voltage output from the sensor may be as high as 1 volt. A lean air/fuel ratio increases the exhaust oxygen content, resulting in a low voltage from the sensor.

In normal operation, O2 signal voltage is routinely varying from almost zero to 1 volt. An O2 sensor signal voltage above approximately 0.45 volts is recognized by the PCM as a rich exhaust; below 0.45 volts as a lean exhaust. The goal of the PCM is to keep O2 voltage moving across the 0.45 volt rich/lean switch point for optimum fuel efficiency and emissions.

The PCM will set an O2 sensor diagnostic code if the sensor does not produce a voltage signal, stays rich too long, stays lean too long, does not switch rich/lean (center too long), or does not switch rich/lean fast enough. OBD-II vehicles may also run PCM diagnostic tests called monitors, which compare and analyze sensor readings to verify proper component operation.

Since OBD-II vehicles may have multiple oxygen sensors located some distance from the engine exhaust ports, these sensors are generally heated to speed the warm-up time period. The HO2S incorporates an internal electric heating element to bring the O2 sensor up to operating temperature quickly (under 35 seconds). Internal heating elements usually operate continuously while the engine is running to maintain an operating temperature of approximately 1292 degrees Fahrenheit to 1472 degrees Fahrenheit. Heated O2 sensors operate at a more consistent temperature and allow greater flexibility of placement locations in the exhaust system.

[astroturf]

WOW, Thats a bunch of info. Thanks, Jim

[phr1$k37]

WOW, Thats a bunch of info. Thanks, Jim

Wow is right. THANKS!
Actually I am trying to find out if with my SG2 if I can pull useful information from the O2 sensors. I think my vehicle is always burning lean so I want to fix it to get some performance .. I mean MPG back ...