Looking for someone to school me on- O2 sensors

[Springthing]

Ok, I've gotten to the point where I've pushed out a few more childhood memories so that means I have room for more stuff!

I'm curious to know if anyone feels like educating those that may not know (like me!) about O2 sensors in regards to GM trucks. Questions I'm curious about are things like..



What do O2 sensors do, in general?
I've got 4 on my '99 K2500, why are there two above the cats and two below?
How often should I be looking at changing them out, preventatively, if at all?
Can there be a need for replacing one without it tripping the CEL first?


Anything else we should all know about O2 sensors?

[trimhalifax]

All I know about them is that they monitor unburned fuel in the exhaust.It helps the computer monitor fuel mixture.whether it is rich or lean and can adjust mixture accordingly.

[trimhalifax]

Here is an article on 02 sensors I found.


The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust as the exhaust exits the engine. Monitoring oxygen levels in the exhaust is a way of gauging the fuel mixture. It tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors can affect the relative richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. There are other sensors to monitor these factors, too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Consequently, any problems with the O2 sensor can throw the whole system out of whack.
The computer uses the oxygen sensor's input to regulate the fuel mixture, which is referred to as the fuel "feedback control loop." The computer takes its cues from the O2 sensor and responds by changing the fuel mixture. This produces a corresponding change in the O2 sensor reading. This is referred to as "closed loop" operation because the computer is using the O2 sensor's input to regulate the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which allows the catalytic converter to operate at peak efficiency while keeping the average overall fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works.
When no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the 02 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to regulate the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. A bad coolant sensor can also prevent the system from going into closed loop because the computer also considers engine coolant temperature when deciding whether or not to go into closed loop.
HOW IT WORKS

The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.
The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors and have no vent hole. It's hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor (for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air). Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail. The difference in oxygen levels between the exhaust and outside air within the sensor causes voltage to flow through the ceramic bulb. The greater the difference, the higher the voltage reading.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system. The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm).
The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal, so many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly. The heating element can also prevent the sensor from cooling off too much during prolonged idle, which would cause the system to revert to open loop.
Heated O2 sensors are used mostly in newer vehicles and typically have 3 or 4 wires. Older single wire O2 sensors do not have heaters. When replacing an O2 sensor, make sure it is the same type as the original (heated or unheated)
SENSOR DIAGNOSIS

O2 sensors are amazingly rugged considering the operating environment they live in. But O2 sensors do wear out and eventually have to be replaced. The performance of the O2 sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce its ability to produce voltage. This kind of deterioration can be caused by a variety of substances that find their way into the exhaust such as lead, silicone, sulfur, oil ash and even some fuel additives. The sensor can also be damaged by environmental factors such as water, splash from road salt, oil and dirt.
As the sensor ages and becomes sluggish, the time it takes to react to changes in the air/fuel mixture slows down which causes emissions to go up. This happens because the flip-flopping of the fuel mixture is slowed down which reduces converter efficiency. The effect is more noticeable on engines with multiport fuel injection (MFI) than electronic carburetion or throttle body injection because the fuel ratio changes much more rapidly on MFI applications. If the sensor dies altogether, the result can be a fixed, rich fuel mixture. Default on most fuel injected applications is mid-range after three minutes. This causes a big jump in fuel consumption as well as emissions. And if the converter overheats because of the rich mixture, it may suffer damage. One EPA study found that 70% of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor.


SENSOR REPLACEMENT

Any O2 sensor that is defective obviously needs to be replaced. But there may also be benefits to replacing the O2 sensor periodically for preventive maintenance. Replacing an aging O2 sensor that has become sluggish can restore peak fuel efficiency, minimize exhaust emissions and prolong the life of the converter.
Unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s vehicles can be replaced every 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications can be changed every 60,000 miles. On OBD II equipped vehicles (1996 & up), a replacement interval of 100,000 miles is recommended.

[cascott325]

what a report! i actually have pondered the thought of replacing mine from time to time, but never have. with 125k + moles, i guess i ought to go ahead and do it. thanks springthing for the question, and thanks trimhalifax for the report

[MrShorty]

Basics: Among your PCM's top jobs is fuel mixture control. The O2 sensor is at the heart of a feedback loop that optimizes fuel mixture. Basic algorithm:

1) Air intake is measured by the MAF. From the MAF reading the computer calculates how much fuel to feed to the engine.
2) The amount of fuel fed to the engine is controlled by the injector "pulse width". That is, the amount of time the injector is held open.
3) The amount of oxygen in the exhaust is read by the O2 sensor, which, as noted, gives an indication of how close to optimum the amount of fuel in step 1. Based on the O2 sensor reading, the PCM "corrects" the pulse width to optimize the fuel mixture. This correction can be referred to as "fuel trim"
4) Return to step one and the loop.

This applies to the upstream sensors. This process gives an O2 sensor signal that should constantly be switching from rich to lean. When the "correction/fuel trim" exceeds a certain preprogrammed threshold, or if the O2 sensor signal stays rich or lean without switching, a fault is indicated.

The O2 sensors downstream of the CAT simply monitor O2 content post cat. If the CAT is functioning as it should, there should be no O2 below the CAT. These sensors are simply to detect when the CAT is going bad.

OBD-2 computers are pretty sensitive to errors. It is probably still possible for the O2 sensor response to be slow enough to not trigger fault code. I've seen some debate about replacing O2 sensors as maintenance every 60k or 100k miles, and others say to wait for them to fail. If I were to change them as a maintenance item, I'd only change the pre-cat sensors, because they are the ones that effect how the engine operates.

[grimreapersshadow]

and a good pointer when puttin new one's in use anti-seize on em or you will end up havin to tap new threads or replacing some tubing and there14 or 18 mil threads special tap for those who didnt know

[Springthing]

Thanks for the great info! Good to know how this stuff works. Much appreciated!

Cascott, good luck with your issue! (sounds like it could be ignition-related to me)

[bry2500]

I love these "school me" posts. Very imformative and great info comes from multiple sources.

Keep pushing out those childhood memories and asking more question Steven.

[JMoney02]

Ah, yes, the dreaded O2's. Very good post here, they play a major part in the life of the PCM and its little functions for maintaining the percise running vehicle. I read this article before, very well written and good advice. A part of the maintenance posts we've had in the past and something that shoud not be overlooked. But now adding the extras becomes a cost factor for some of our Trucks/SUV's, they should put out a package deal for some of us.......Thanks Springthing....Oh, wait till you get to 50yrs, your memories will be where are my glasses now,Ha.....

Jeff