P0300

nickb2

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#31
The B1S2 O2 and B2S2 O2 sensor fuel trims are stuck on 99.06 and not moving, What does this mean?
It means you have a shorted or open circuit for those two aft sensor. Or, you have two blown aft o2's that went at around the same time. Not unheard of, they do after all usually have the same moleadge. Kind of like a pair of shoes. But in automotive, someone could have swapped one and you may not know. Any, @ 99%, complete o2 fail. PCM algorythm goes to 99% by default. It does this to flash a flag and light the check engine. That monitor did not pass. Of course it won't But that is the default mode of mode 6.
 

nickb2

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#32
In mode 6, you see the pcm doing its logic. How it passes and fails pre programed decision for highly tuned cars and trucks we drive now.
 

nickb2

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#33
When you see trims like 99% on both banks, dead give away the pcm is now in limp in mode. Data proves it. Cuz if a car could show 99% leaning or enriching in real time and NOT be in limp in mode, engine would not idle or run.
 

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#34
When you see trims like 99% on both banks, dead give away the pcm is now in limp in mode. Data proves it. Cuz if a car could show 99% leaning or enriching in real time and NOT be in limp in mode, engine would not idle or run.
There are rear O2 sensor TRIMS.....In 45 years I have never heard of rear O2 trims. I can't find any info on them. That's why I asked if his software explained them and how to interpret the readings. I'm at a total loss with these numbers. My best bet is it's not trims at all but possibly the percentage of a level the system expects with 100% being perfect. That would make more sense. If the rears are doing their jobs well and the cats are storing O2 well we may expect this number to be 100%. Meaning the system sees a very good catalyst. I don't know what else it could possibly mean.
 

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#35
I have no P0161-164 codes. Only the P0300.
The data grid that I posted has green, yellow, or red bars that are displayed under the numeric value.
 

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#36
Both sensor fuel trims are maxed out at 99% rich, right?
That's not how fuel trims work Normal ft are in the 5% range and 10% is getting to the high side whether + from lean condition or from - rich condition. They only go to 25% correction and this is max and sets the appropriate code. The O2 sensors are used to calculate the rich lean and are not really trims. What the computer has to do to get them into fuel control is the trims. If they are not able to achieve fuel control they will set a code for trims either high or low. I have never seen a code with a definition of O2 sensor trim of any percent. There's something not right about that software. Forget the trims as this thing is in good fuel control if we can believe what the scanner is telling us. Multiple misfires will nave skewed trims as the raw fuel and oxygen will be causing fuel control problems wich will make your trims way off from 4 or 5 which if this is accurate I don't think you have a lot of misfires going on. How is it running now?
 

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#37
I drove it to work today. 25 miles each way. The miss is hardly noticeable and not constant. It pulls smoothly.
The SES light set and flashed several times at me. My hand held code reader (not the Giotto laptop software) showed a P0300 only.
 

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#38
I drove it to work today. 25 miles each way. The miss is hardly noticeable and not constant. It pulls smoothly.
The SES light set and flashed several times at me. My hand held code reader (not the Giotto laptop software) showed a P0300 only.
Have you filled it with new fuel? You may want to have an injection cleaning done.
 

nickb2

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#39
There are rear O2 sensor TRIMS.....In 45 years I have never heard of rear O2 trims.
For sure there are. Many cars I see in my bay have pre cat sensor/s gone out and PCM can still correct because the rear is working. Hence proof the rears also have a trim strategy involved there.

Not all makes and models have the rear trims involved, that is basically up to the manufacturer. It is very hard to find info on how they do that, it is a secret of sorts they don't want to disclose.

I read that your have been around this stuff for 45 years, so alot of the diagnosing you have done was under simple "direct feedback" logic.

Some of what I write here comes from different sources, personal experiences and the various courses I attend.

What I write here is from some sources that write better than me in english, but they do tend to substantiate what I said before.

So the newer tech is now using what is commonly called "feed forward" using neural network logic.

The values we were seeing in Gm guy's snapsot for the rear trims was what is called an "additive value" Which has a % range of -100 to +100%.

Multiplicative values are usually reserved for upstream o2s and typlically have a value range of -40 to +40, and additive values for downstream % range as written above. I am uncertain, but pretty sure the following statement of mine is true, "Bosch" perfected this system and many of the algorithms in the gms of that era and many now are still based on Bosch strategy. Probably a major patent there. o_O:money:
 
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nickb2

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#40
My thoughts for reading those values as a shorted or open circuit feeding both rear o2,s is that both went to a max identical value. But that is just an assumption. Normally, this type of situation would have produced a few other codes such a low or high voltage circuit codes.

Although I am not saying this is the case, I have seen a few ECM's with such values and it ended up being a bad ecm.

The other day, on a gmc k truck with a bosch ecu, the IAT pid data was -400something Celsius or farenheight (can't remeber which scale my scanner was on, I switch it sometimes to celcius to better suit the newer younger guy's who can't figure out the conversion from f to c.):eek:,

Probe wiring, was good. For laughs, pull an IAT from another truck in the parking lot, same value. Opened the ecu, could find corrosion on the main PCB, which I assume was the trace for that circuit, So the ecu went to a default value of -400c/or F. This diesel was obviously in trouble. I called a used ECM, programmed it with newest software and set mileage and anti-theft, was good to go.

So sometimes, these values we see in mode 6 which don't show up elsewhere help to pin point a problem. Obviously, this truck can not run well with those types of values hence the missing and flashing light. This truck is missing more than gm guy is letting or he is just not noticing it since a v8 does run smoothly when missing on miltiple cylinders if they don't all miss at same time even if that light is flashing. The ecu is seeing catalytic imminent danger of failure.
 

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#41
I just removed the spark plugs for examination and they're all new, light tan and appear to me to be in vary good condition.
They're Champion 7940, I'd rather have AC's, but they should be ok for now.
Have you filled it with new fuel? You may want to have an injection cleaning done.
Yes, I filled it today and added a bottle of injector cleaner, Not the same as what you recommend, but I'll look into that
 

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#43
My thoughts for reading those values as a shorted or open circuit feeding both rear o2,s is that both went to a max identical value. But that is just an assumption. Normally, this type of situation would have produced a few other codes such a low or high voltage circuit codes.

Although I am not saying this is the case, I have seen a few ECM's with such values and it ended up being a bad ecm.

The other day, on a gmc k truck with a bosch ecu, the IAT pid data was -400something Celsius or farenheight (can't remeber which scale my scanner was on, I switch it sometimes to celcius to better suit the newer younger guy's who can't figure out the conversion from f to c.):eek:,

Probe wiring, was good. For laughs, pull an IAT from another truck in the parking lot, same value. Opened the ecu, could find corrosion on the main PCB, which I assume was the trace for that circuit, So the ecu went to a default value of -400c/or F. This diesel was obviously in trouble. I called a used ECM, programmed it with newest software and set mileage and anti-theft, was good to go.

So sometimes, these values we see in mode 6 which don't show up elsewhere help to pin point a problem. Obviously, this truck can not run well with those types of values hence the missing and flashing light. This truck is missing more than gm guy is letting or he is just not noticing it since a v8 does run smoothly when missing on miltiple cylinders if they don't all miss at same time even if that light is flashing. The ecu is seeing catalytic imminent danger of failure.
If both downstream O2's have failed, why do I not have a P0161,2,3 or 4 code? I'm not trying to say you're wrong, just trying to understand the system. Now, the ECM has some corrosion on the outside of it's aluminum case, and the truck did have a tarp covering the engine for a couple years, so the ECM may be in question, but it genuinely does not have a consistent miss and does not run roughly.
 

nickb2

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#44
Another example of where long term and short term, pre and aft o2s sensor is critical. I had a buick lacrosse in today, with multiple misfires on all cylinders and for some reason, not cyl#2, and yes a p0300. But also a fuel pressure regualtor performance problem. These new GDI's are known for camshaft failure and lobe for FP will cause this situation. But this was not the case. So I did a complete GDI injecotr cleaning.

Trims all across the board went from -37%ish to almost zero in the time it took to do the three stepd GDI cleaning proccess. I am keeping the car for a few days to make sure all the IM monitors pass and the client can go away with the car knowing this is fixed.
 

nickb2

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#45
Yeah, like I said, it was just a thought. But I don't think that scanner is lying to you. Those trims are obviously not what you want to see, much less the ECU.

So the next thing we want to look at is aft o2 sensor switch. Can the scanner see that live? I think it can. If I believe the mode 6 data, they are not switching the way they are supposed to. So this truck will never pass the o2 monitor. Will always have a CEL light on and not pass emissions.

I am in no way telling to throw parts at this truck, but mostly instructing you, not telling you, that further diagnosis of the o2 sensor system is needed. For a 1996, they had a pretty good setup, could code different conditions, but one thing these older trucks cant't do that the newer ones can is flash a code that actually tells you if a short or open is present in the circuit.

So one thing you can do, is probe the rear o2 circuits to the ecm. I will provide the wiring for that if your so inclined. Once you have done that, it narrows down the options, which is a good thing.