--Blue, let me phrase it like this. I am the ultimate cheap *** when it comes to fixing any of my own cars. I have never randomly guessed at parts being i hate to be wrong and have to replace two parts when only one was needed. With that said, if your current problem was present on my vehicle, being a post cat o2 code and no missfire or additional dtc's set, i would have thrown a post cat sensor at it more than likely without even scanning it. This is primarily based on the facts that with that dtc as the only one set, 99% of the time its going to be the sensor. Cats do fail but its rather rare and normally due to the vehicle being driven in a failed state for a long period of time. If you can deal with the possibility of a wiring problem and replacing an un needed sensor, then go ahead and hang the sensor without going any further. Outside of that, if you dont have anyone to scan it for you and diagnose it properly, you would prob end up paying the 70-90 one hour diag fee at the dealer. The only good thing about that is if the cat was bad, and your vehicle was still in emissions warranty, they would replace it and diagnose the vehicle at no charge. I would be willing to put money on this though, if you took it to the dealer you would be told it needs a post cat o2, and if the light was still on, they would later warranty the cat after you were charged, at that, more than likely un benounced to you.

--True blue, dont beat me up on this statement but..... if you dont know or understand all of parts of the theory just ask and im sure someone here will explain it. When you try to diss credit someone with information that is blatently erroneous you will embarrass your self along do a diss service to anyone who is reading these posts and trying to understand why a particular repair needs to be made. Yes i have seen cats get too hot from lean conditions, though some deform and crack, most, even if overheated will still perform and function as designed. Any missfire, rich condition, or even a severe fuel leak into the intake (stuck injector) will severly damage a cat if the engine is opperated in this state for a period of time. Actually, not counting the back in the day junk ceremic cats, its is the most frequent cause of cat failure. As for a post cat o2 setting a switching code....it can set a high volt, low volt, or heater dtc. If you have ever watched the actual voltage of post cat o2's they rarely switch at all, even when hot. The pcm is looking at the average voltage of the pre cat o2 and comparing it to the post cat o2 to determine if the cat is functioning. Normally the difference is notable being most all unburned hc's are burned in the cat, this is why you wont see a lean/rich switch like a pre cat o2.

My 2¢'s

 

Hmmm rear oxy sensors rarely switch at all-Not what I see on NGS or WDS then why do Fords have codes for 12 or 22 oxy sensors not switching.I don't do GMs so can't compare- do know that lean codes on my brother-in laws 98 K-10 ended up being bad fuel pump and not oxy sensor local GM dealer diagnoised Do know for a fact that cat effic. codes on Fords are not repaired by replacing oxy sensor, cat replacement or pcm recalibration is only way I have ever repaired them Tsbs and Ford spec. service bulletins verify this.Never said I know everything(I just think that sometimes)

 

Im not sure i explained that right. The switching of the pre cat o2's is the pcm lag time between cycling lean/rich. It will see rich, (o2 high volt) command less pulse width, at that point it will be lean (o2 low volt) and command more pulse width. Just a constant cycle to maintain the supposed 14.7:1. When you think of how much unburnt fuel, hc's, are burnt in the cat, (gass sniffer in the pipe of a new vehicle, hc's almost dont exist now days, at least extremely low when hot) there is not a notable lean/rich switching that can be seen in voltage. There is so little fuel left its hard to get a high voltage read. Dont get me wrong, they post cat o2's do fluctuate, but its no where near as notable as the lean/rich switching on a pre cat sensor. Those switching dtc's are just high/low volt errors. Not switching is referring to the pcm seeing a reference only and determining the sensor is open, or seeing 0 and determing a short.

My 2¢'s

 

Thanks again guys for the help, and helping me understand the process of how the PCM uses the O2's. Didn't mean to start a ****in' contest, but you both have been a big help to me. Just hope that I can pass on some knowledge at a later date.

 

Whew! I just joined this forum. I understand Oxy Sensors and I almost got confused reading all that. Simply put, Bank 1 refers to the side of the engine that cylinder 1 is on. On a Ford that should be the Passenger side bank. Whenever there is only one sensor, it is refered to as bank 1. Sensor 1 or 2 refers to whether it is the first in line after the engin or second. The sensor in question was Bank 1, Sensor 2. There is only one downstream (sensor 2) so it is refered to as bank 1. Upstream sensors (banks1&2 Sensor 1) should cycle between approximately .2 to .8 volt about once every couple of seconds. They all will vary a little, but it they only cyle once or twice in a 10 second period they are refered to as 'lazy'. These sensors tell the Control Module whether the engine is rich or lean. They control module cycles the engine a little lean to a little rich, back and forth which is why the sensors pick that up. The Catalytic Converter is supposed to burn this extra fuel/air so it doesn't go out the tailpipe. So, the sensor in question only monitors what is coming out of the Cat to see if it is doing it's job. If the Sensor 2 cycles fast like the two upstream sensors, the cat isn't doing it's job. It should cycle very lazy, about once every ten seconds. If it is not cycling at all, it is probably the sensor or the wiring. You can manually hook a voltmeter to the wires on the sensor and run the engine, (the engine will have to be warmed up). Watch the volts that the sensor in question is putting out. It shoult slowly go from about .3 to about .6 volt. A steady reading, (no real fluxuation) indicates faulty sensor/wiring.

 

Fixitnow-Thanks for the explanation! Is there a difference on which wire I should hook up for the positive and negative on the wires to do this voltage check? IIRC, there are three wires coming out of these sensors. That sounds like a very easy test to see if the sensor is working correctly.

 

Three sensor wires usually have one ground, one signal, and one heater. The ground you can find by checking continuity to ground. The signal will show less than one volt with the engine warmed up. The heater will show more volts. I'm not positive, but I think the wire that goes to the heater should show a full 12v. This heats up the sensor so that it can start working sooner. Oxygen sensors cannot "see" oxygen unless they are warmed up, (well over 100 degrees). You need to connect to the signal wire and the ground. If you have a digital multimeter, it doesn't matter which to which, it will just show negative voltage instead of positive if you have it backward.

 

I though I might need to define cycle a little bit for you. Every time the voltage crosses 0.450 Volt and back, that is a cycle. If the post cat sensor cycles 2-3 times in a 10 sec period, it is probably ok as long as the voltage gap isn't real big, should not be going all the way to .8 and .2 volt. Oxygen sensors can be tricky and fool you, but this is the way they are supposed to work. Catalytic Convertors are not supposed to go out. They do sometimes burn up, the most common cause is an engine misfire, so much extra fuel and air that the Cat can't keep up with burning it all and melts down. On OBDII vehicles (96 and up) if the Check Engine or Service Engine Soon light flashes, that means there is emminant Catalytic Converter damage, stop driving. If you do this voltage test and it seems like it is responding correctly, it can be a couple of different things. It could be the PCM program like someone said earlier. Also, sometimes it works fine when you test it, but going down the road it will quit, Sometimes the heater on the sensor will go out. When you are driving down the road, the air passing under the vehicle will cool the sensor down too much and it won't be able to "see" the oxygen. The post cat sensor is more prone to this because it is so far from engine, it's only other source of heat. If you think that the heater is out, there is no fix for the heater, replace sensor.

 

I'l do the testing on it this weekend to see if I can find the sourse of the problem. If it matters, the CEL came on each of the two times when I was sitiing still, waiting in line with the truck in Drive.

 

I hope I didn't confuse you, either, with my similar problem as I'm kinda "in the dark", too. I've heard several reasons why my problem arose, from too much mud caked up underneath the truck overheating the exhaust system (cat) to bad fuel. THe code first started when I had to use 86 octane fuel instead of the 87 I have at home, and have also had several coolant leaks, 2 busted trannies, etc. The mileage is about 120K, so no warranty. The reason I plan to replace the O2 sensors is since I am going to get a new cat and muffler installed, it will be a good time to change the rear sensor. My advice is still the same, to replace the 2-cat stock system for a single cat. But that is up to you whether or not you want to spend $500 to fix a minor problem!!! I am looking for a little more performance and sound in my case.