Do they go from 100% to 80% to 30% working or are they on or off.
Mech told me I should change sensors even ifthe engine light isn't on
Make sense ??????

 

Is there any back story to this question???

Oxygen sensors will decrease in activity over their life. This is what will lead to a check engine light as the sensors get lazy. The initial activity of a sensor is a direct reflection of how long the sensor will last. Also, a lazy O2 sensor can cause mileage problems before triggering a check engine light.

If your vehicle has over 75k miles, then chances are the O2 sensors are not as active as they were when the vehicle was new. This could be lowering your mileage even if a DTC is not present for the O2 sensors. Only the upstream sensor(s) (the one(s) closest to the engine) affect the fuel delivery system. Any additional sensor(s) are catalyst efficiency monitoring sensors only (depending on the year of the vehicle). On the mid'97+ trucks, there are 3 O2 sensors. For the '95's, there are 3 (one upstream and 2 downstream). On the '96-mid'97, there are 4 O2 sensors (2 upstream, one right after each exhaust manifold, and 2 downstream).

Before I would replace any O2 sensors, I would want to see their activity either on an oscilliscope or on a full feature scan tool. But if you are over 75k miles on the original sensors, you may be in for a replacement. If you want the new ones to last like the originals, go with the OEM supplier - AC Delco!

 

Word of advice, get an O2 sensor socket to make it easier to get the sensor off.


Or you can do what I did this weekend, and cut the end off with a dremel and some pliers so you can get a regular deep socket on it.

http://www.pewterz28.com/forumstuff/dremel.jpg

 

Mine are definitely overdue. I'm at 165,000 miles, and I'm not sure they've ever been replaced. They definitely haven't been replaced within the last 75,000. And yes, I'm experiencing the systems described above.

So here's my question. I have a '96, with the four-sensor system. I only want to by AC Delcos, but these things are friggin' expensive! AutoZone and Advance, the two biggest parts stores in my area, don't carry AC Delcos. If I go to a stealership, I could be paying close to $400 for a set, if not more. Does anybody know of any good vendors? I've run some online searches and have come up with nothing but Ebayers, and they generally don't warranty their products. Thanks again for the help.

 

RockAuto usually has AC Delcos, havn't checked O2 sensors though.

Edit: RockAuto has AC Delco O2 sensors for my 2000 at $58.79

 

For your '96, the ONLY ones you should be looking at replacing are the upstream sensors - the ones closest to each exhaust manifold. To replace the downstream sensors without an associated DTC and proper diagnosis is a total waste.

I have an O2 sensor socket and never used it once on my Bravada. Access is pretty good. I just used a liner wrench of the appropriate size.

 

Others who are interested -- these are the ones Wolf's referring to...

ACDELCO Part # AFS105 {#25312200}
SENSOR,HTD OXY(POSN 3) POST CONVERTER
SENSOR,HTD OXY(POSN 2) PRE CONVERTER
SENSOR,HTD OXY(POSN 1) IN EXH MANIFOLD/CROSSOVERMore Info$58.79$0.00$58.79

So why is it a waste? You don't think they should be replaced after 165K miles? The thing is, they trip a code, but it only happens after I've been driving hard. Then it goes away after the second or third time I drive the vehicle again. It's annoying because the code goes away before I even have a chance to get a scanner on it. However, I'm almost willing to bet that it will be an O2 sensor code when it does come up. There is nothing wrong with the driveability, other than that sometimes it hesitates when at cruising speeds. And I definitely have lost fuel economy.

 

He was just saying it is a waste to replace the downstream sensors (since they don't affect fuel delivery or engine performance). Only replace the ones closest to the manifolds unless you are actually getting codes for the downstream.

 

Ok, I'm new to this forum, but I'm also new to troubleshooting exhaust faults. I'm getting two codes, PO147 - o2 heater circuit bank 1 sen 3 and PO151 circuit low voltage bank 2 sen 1. I have replaced the final sensor, sensor 3 that is, located behind the cat converter. It was cracked and I figured this was an easy fix. It didn't work. My engine symptoms are=
Service engine light and the two ODB codes above. In idle, vehicle acts normal, no problems. In drive however, at around 2k RPM's it appears that I get a lean mixture and my engine begins to die, it will chug but not totally quit, also a bit of a knocking sound (lack of fuel???). Is this a normal symtom of my system being in OPEN LOOP, or does this show me I have other problems beyond my OX sensors? The bottom line is that my engine acts completely normal up to 2k rpms in drive, then it knocks and stalls. What should I try here? New fuel filter? Clean injectors? Replace my foward OX sensors? (and which one? Sensor one? or does PO 151 just tell me I'm getting a lean mixture??) As you can tell, I have lots of Ideas, lots of answers, and little money. Thanks for your help. -Brett

 

Psychropod,
The down stream O2 sensors are used for catalyst monitoring only and do not have anywhere near as tight of constraints (activity wise) as the upstream sensors. And due to the nature of the job they are doing, high activity isn't crucial to the function. As a result, you don't replace them until you specifically have a code stating a problem with one of them. At which point you first reset the code and wait for it to come back. If it does return, you swap the pre- and post-cat sensors to see if the code follows the sensor. If it does, then replace the sensor.

With the upstream sensors, activity is crucial to engine operation. The PCM adjusts the injector pulse width many times a second based on the readings from the O2 sensors. If the O2 sensors become lazy, the PCM gets delayed or choppy data, causing the PCM to be slow to adjust the IPW. Sooner or later, the O2 sensor will be lazy enough to cause too long of a time period between the lean to rich transitions and set a code.

Brett, I will respond to the new thread you created. I moved it into the 2nd Gen Tech area though.