Determine which output(s) are incorrect.
This post is part of a series describing the generic, high-level troubleshooting process.
Previous Posts:
What is Troubleshooting?
Simplified Troubleshooting Overview
Step 1. Understand the correct inputs and outputs of the “black box.”
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Step 3. Figure out which input(s) affect that output(s).
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 2. Determine which output(s) are incorrect.
Now that you understand what the input(s) and output(s) are, and what they are supposed to look like, you can compare the expected output(s) to the current output(s) for the current input(s). If one or more outputs are not what you are expecting, then you have a place to start looking. Make sure that you check all outputs, often more than one will be wrong, which could be an indication of multiple problems, or could help point you to a single problem that affects some or all of the incorrect outputs.
When possible, change the input(s) and recheck the output(s). The fault may only happen on certain inputs, or only on a specific combination of inputs. If you know an input configuration causes the fault, make certain that you set the inputs in that way and check the output(s). Do not only check that one configuration, it may have issues with other input settings as well. The more information that you have on the current vs. expected behavior of the “black box,” the easier your troubleshooting will be.
If multiple errors seem to point to multiple problems, or you’re not sure, take note of all of them, but pick a single fault to work on first. It is easy to get lost when working multiple issues simultaneously. Troubleshooting is not a place to try to multitask, it will end up taking longer than if you just focus on one issue at a time.
When one of the incorrect outputs is an input to another “black box,” disconnect it from the other box, if possible. It is not uncommon for the input side of the next box to have an issue which appears to be a problem with the output of the first box. Often, when this is the case, simply disconnecting the two boxes will then give you the correct output on the first box. If this happens, you need to troubleshoot the second box for that issue, not the first. Congratulations, you just saved yourself hours or days of looking in the wrong place.