Step 3: Troubleshooting Overview

Figure out which input(s) affect that output(s).

This post is part of a series describing the generic, high-level troubleshooting process. It is not intended to diagnose, treat, prevent, or cure any disease.

Previous Posts:
What is Troubleshooting?
Simplified Troubleshooting Overview
Step 1. Understand the correct inputs and outputs of the “black box.”
Step 2. Determine which output(s) are incorrect.

Next Posts:
Step 4. Verify that those inputs are as expected.
Step 5. Analyze the path(s) from input(s) to output(s).
Step 6. Check all connections.
Step 7. What are the devices in those path(s) that change the data?
Step 8. Eliminate the devices that could not cause this output.
Step 9. Test the remaining devices.
Step 10. Repair/Replace faulty device(s).
Step 11. Fully test the system.

Step 3. Figure out which input(s) affect that output(s).
So now we know what the “black box” is supposed to do and which outputs are not correct, the next step is to start diving into the inner workings of the system. We need to determine which input(s) are supposed to influence the output that is being troubleshot. The key word is “supposed to.” Since the system is not functioning properly, testing may not give us the correct results, so we need to analyze the system. Hopefully you have schematics, or a wiring diagram, or something to make this easier, if not, you will need to dive in and trace the electron flow yourself.

See how you do with this made up diagram below. The connecting dots are one-way, everything coming in from the left is combined and output to the right. Anything on the right, does not affect things to the left. Which input(s) affect which output(s)?

How did you do?
Now lets get more specific.
Output “W” is not functioning correctly.
Which input(s) influence the output at “W”?
Can you tell?
Need a hint?
Sorry, I can’t help you other than to let you know that the answer is below.
Mainly, at this point, I’m just writing lines hoping to push the answer far enough away that you don’t accidentally see it.

The answer is…
B,C,D, & E
Did you get it right? Take a look at the image below to see why. The pink lines are where I traced backward from “W” to the inputs.

I wish real world systems were this easy, but they are not. Mainly because the “dots” are relays, PLCs, sensors, electronic circuits, even full circuit boards. This makes the paths much harder to trace, but it needs to be done, to enough detail to find the needed answers. There is no need to determine what the “dots” do or how at this point, just that they connect input to output.

One other thing to check is, do any of the other inputs change the output? In this case, does “W” change when “A” changes? It shouldn’t, if it does, that is another clue for your troubleshooting. Consider everything that you learn (no matter how small) to be a clue to the puzzle.

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