P0300

nickb2

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#46
Back in the day, even the gm tech 2 scanner could not see mode 6 data (there wasn't even a menu option for it), and even if it could, the common person could not decipher it as it was all in hexidecimal format. But since then, the scanners we use can now interpret that data into something the general humana/erectus hominus, not a robot, can understand thanx to the good ppl like the NASTF for example. If your a lobbyist for the auto industry, eat my s&%t That is why your seeing that old hex data now in percentage form. This is of great value in this triuble shoot. It is a GREAT starting point.
 

GM Guy

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#48
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.
Great! I was just thinking about my next step, troubleshooting the downstream O2 sensors. I have a good quality Fluke multimeter and know how to use it. I was wondering if there is some way to simulate the mV signal back to the ECM to make sure it is responding correctly to a viable O2 sensor.
 

nickb2

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#50
I was wondering if there is some way to simulate the mV signal back to the ECM to make sure it is responding correctly to a viable O2 sensor.
Yes there is. But before, I am now confused. Your IM readiness tree say o2 monitor completes. So now you just need four the other monitors complete but they are not supported. Also, even more confusing, the tree says the misfire monitor passed/completed. Yet your hand held gives you a p0300. If the last screen shot you posted is the true one of how this truck runs right now, there is no miss at all.

What am I supposed to be looking at? Which snapshot is of the current way the truck is running right now?
 
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nickb2

Wrench. Diagnostic Tech.
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#51
As for the trick question for fooling the ecm. A dummy o2 sensor can be setup to send a switching voltage which will mimick what it wants to see. Watch these two videos very closely.


 

GM Guy

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#52
Yes there is. But before, I am now confused. Your IM readiness tree say o2 monitor completes. So now you just need four the other monitors complete but they are not supported. Also, even more confusing, the tree says the misfire monitor passed/completed. Yet your hand held gives you a p0300. If the last screen shot you posted is the true one of how this truck runs right now, there is no miss at all.

What am I supposed to be looking at? Which snapshot is of the current way the truck is running right now?
I think I took that screen shot after I cleared the code and was sitting in my garage. Those checks always say passed, I think....
Tomorrow I'll start the cycle again. This time I'll pay more attention the this mode 6 screen when the SES is lit. When checking the plugs this evening, there may have been 1 plug wire that wasn't as secure as the others- The previous owner installed the new ignition parts- And the test drive afterwards did not yield a P0300- too soon to cal it fixed. I need to look at the fuel trims again to see if they are still the same 99.06%.