I read a lot of threads where we exchange opinions and tips on OBDII scan errors with sometimes frustrating results. In order to address this let me expand a bit on what OBD is and is not able to tell you. The following has been expanded from a different thread.

I would like the SAVVY techs with real life experience to participate and expand on this thread which can give us Blazer users a better insight into our cars.

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The following is for information and should in no means be a discouragement, but a help to make better, more informed decisions.

Concerning OBD II and some misinterpretations:

A general misinterpretation stems from the name itself (On Board Diagnostics) which is in real life only half true. Later in this thread.

OBDII has been implemented as an advanced protocol over OBDI (obviously) and has two main reasons to be there:

1st the emissions laws mandated a means of on board self diagnostic to tell if something is wrong with the emissions controls.
https://en.wikipedia.org/wiki/OBD-II_PIDs

2nd the U.S. legislation mandated an access to all vehicles via a standardized connection. (IMHO something related to the anti trust laws). All foreign manufacturers have adopted the OBDII connection in some countries as well due to anti trust laws in some just out of convenience.
https://en.wikipedia.org/wiki/On-board_diagnostics

Before 1995 GM called it's diagnostic connection ALDL (Assembly Line Diagnostic Link) which is sometimes referred to as OBD I but is not fully correct as the OBD is a set of protocols and standards while the ALDL is a factory access to the control units.
https://en.wikipedia.org/wiki/ALDL


Despite it having the name of "ON BOARD DIAGNOSTICS" What can OBDII really do?
It will give you the required standard output codes defined in the OBDII standard. This means that the computer is passing along the information required by the OBDII - this is the only thing that the control module will show a generic OBD scanner.

It will, apart from the sensor data which can sometimes be read in real time while running, not give you any additional troubleshooting or analytical data.
That is: The computer may have a much better idea of what went wrong with let's say the reason for the stalling but is not telling you! Instead it may or may not trigger a "CYL #4 MISFIRE".
You think: Gee - my spark must be bad for the misfire. But what the computer is not telling you, is that it could also be the injector itself which is out of spec in resistance. Instead it gives you the generic "MISFIRE" code.

When you hook up a factory computer (most of the time on the same OBDII connector) and the guy/girl using it is competent it can tell you a lot more than that. The factory equipment will normally send a code (Security Seed) to the control units on the car which identifies it as factory computer and then the control modules start to be a lot more talkative then with a generic OBD scanner.

Take your ABS harness. The computer will throw a generic OBDII code for the ABS - let's assume it throws "C0040 - Right Front Wheel Speed Circuit Malfunction".

Does this identify the problem? Well in part maybe...
... But from a technicians standpoint, no it is not telling me what the problem is - there is more to it.
Modern - let's say something from 2000 onward computers will tell the factory diagnostic computer a lot more than that but will just pass the above DTC to the OBDII connection. Also between 1995 and 2000 the control module may tell a factory diagnostic also a lot more of what has happened for it to trigger a fault code.

The factory diagnostics can tell if the wiring is broken. It can tell if the ABS control module has an internal fault. It can tell if you have a signal but it is weak (e.g. a bent sensor support on the wheel). It can tell if you have an intermittent signal (e.g. same as before or a short or bad connector). Depending on the wiring and the diagnostic capabilities you can run active tests with the factory testing equipment. The computer may also be able to tell if the sensor is out of spec (e.g. reads too high or too low resistance/current/voltage).

Not sure but I guess that from 2000 a factory diagnostic should also have a troubleshooting guide built-in. This will tell you what steps to take one by one to come up with the correct diagnostic. Assuming the above ABS code it may tell you to disconnect the connector in the wheel well and measure for resistance. It will also tell you what the resistance should be. Then it will tell you to disconnect the connector at the ABS control module and reconnect the wheel sensor and measure the lines from the module to the sensor, again with the expected values.
Within 5 minutes or so an expert may be able to tell what is wrong.

With some problems which are hard to come by I would relatively quickly consider a GM dealerships which has the correct diagnostic and have them make a preventive on the diagnosis itself and after that, for the repair and parts needed.
Sometimes you pay 50$ for the diagnostics and it's printout and it's worth it's money.
Mind you - I'm talking a dealer which has the GM factory diagnostic and not an aftermarket scanner as good as they may be.

I take care of the testing equipment in a garage specializing in Italian sportscars. The scanning equipment we have is apart from being able to talk to the specific brand of cars also a built in OBD II scanner. It can read my SAAB as well as it does one of the sportscars.

The difference in what the OBDII tells you vs. the factory diagnostics may be the following:

Situation: We had a car showing the MIL and going into limp home mode after a while. When cold it would run just fine.
Model was from 2009 quite advanced convertible.

OBD II would throw a MIL and P0060 Catalyst temperature, bank 1, sensor 1.
The generic OBD sees 3 control modules of which it can access 2. They are 2 BOSCH Motronic engine control units and an ABS control module which cannot be connected to via the standard OBD port. The ABS has an additional OBD diagnostic port in the engine bay. But with a generic controller we cannot access this box.

What is causing this? Could be a lot of problems, leaking injectors, waste gate not opening under boost, fresh air injection not shutting off...

Then we hook up the factory diagnostic and we get:
Access to about a dozend control modules! I will call them CU's from here on.
2 BOSCH Motronic engine control modules (ah we knew these two) obviously the ABS and now we have access to more:
Convertible CM, LH seat CM, RH seat CM, Dashboard CM, Main Node (Security and alarm system) CU, Car disabilitator (security the one which acts on inputs from the alarm system) CU, Radio CM, LH Cat CM, RH Cat CM, HVAC CM, Bilstein Suspension CM, and a couple more I don't remember.

Now we see about 6 faults related to the catalytic converter overheating. Still it is no trivial work to come to the source of the problem. We see the cat overheat, injector faults, ignition fault, reversion to low power limp home on the Motronic.
And we see something very interesting! Internal fault in the LH catalytic converter temperature control module.
In order to analyze if the thermocouple (the actual sensing element) is bad, we swap the LH and RH box.
The fault after a test ride changes side. We have identified the error, replaced the control module and start taking care of the other faults.
It will show in the car's diagnostic and electrical manual, that these errors are related to the malfunction of the temperature control module.
As the car can no longer tell if the cat will overheat it cuts back on ignition and injection. The Motronic will throw an error because it tries to do it's job but cannot because of a safety logic. The ignition is the same.
Cancel all errors and go for a test ride. All back to normal.

I can read and log via the real time data link from the diagnostic port about 1'500 possible faults and about 300 parameters of the car such as temperatures, injection cycles, ignition timing and so on. Things that may or may not show with a generic OBD scanner. But more important I can also see the setting of valves such as EGR or canister purge valves. I can see circuitry resistance, circuitry activation and read data from the control modules. I can see the cam sensor position, the fresh air injection, fans running.

More importantly I can do active diagnostics such as switching off injectors to test them. (No drop in rpm = faulty injector) etc.

This is a bit of insight into car diagnostics and what the manufacturer will not tell us. I for myself have all three factory manuals for my 1994 S10 Blazer including the electrical & diagnostics. Unfortunately still trying to get my ALDL to work with my laptop but that is because I spend too much time in another garage.

I hope this helps clarify a bit about OBD II generic scanner readings and the real faults.

 

I think the important things to take away from all this is that

1: Not all OBD2 scanners are created equally
2: There are many scanners with capabilities in-between fully generic scanners and the Tech 2 - and you must understand exactly what yours is capable of.
3: Scanners are only a diagnostic aide. They cannot tell you what part to replace through a printout.
4: There can be no substitute for understanding exactly how a system works - and then devising a rational way to diagnose it. I have done this many times because I did not have a TECH 2 to do a specific factory diagnostic flow chart. If you can understand what they are trying to accomplish in the diagnostic chart, there is usually another way to do it.

 

Great additions! Thanks for sharing.