A Later Modification Caused the Fault On Your Install: Decision Tree

Why this matters

You suspect a change someone made after you is what broke your install. You are probably right, but "probably right" is where a lot of techs get burned. A modification sitting near a fault is not proof it caused the fault. If you tell a customer their handyman's change is to blame and it turns out to be your original work, you lose the argument and the relationship. This tree is how you prove it, so your finding holds up.

Start here: is the modification itself dangerous

Some modifications are a hazard on sight, and that outranks the diagnosis. If a later change defeated a safety device, undersized a conductor or line, bypassed a relief or backflow protection, or created a combustion or shock risk, make it safe first: de-energize and verify dead, relieve pressure, or shut the system down. Then document it and continue. A dangerous modification is a stop-work finding regardless of whether it caused the current complaint.

Step 1: locate the fault precisely

You cannot tie a fault to a change until you know exactly where the fault is. Measure it down to a component or a section, not a general area. A vague fault location produces a vague, arguable conclusion.

Step 2: is the modification actually in the fault path

Ask whether the change physically or electrically sits in the chain that produced the symptom.

• If the modification is upstream of, at, or feeding the fault, it is a live suspect. Continue.
• If the fault is in a part of your original work the change never touched, the modification is a bystander. Do not pin it. The fault is elsewhere, possibly yours.

Step 3: does the change violate a rating, spec, or code

A modification causes faults in predictable ways. Check the change against what it should have honored.

• Wrong size or capacity: an undersized wire, pipe, or breaker for the load; a mismatched part rating.
• Wrong material or method: an incompatible material joined to yours, an improper connection, a missing support or protection.
• A defeated or altered control: a bypassed switch, a changed setting, a removed safety.

If the change breaks a rule and the symptom is exactly what breaking that rule produces, your case is strong. Note the specific rule and the specific symptom together.

Step 4: test the causal link, do not just correlate

This is the step techs skip. Prove the change is driving the fault.

• Remove, bypass, or restore to original where it is safe to do so, and see if normal operation returns. A fault that clears when the modification is undone and returns when it is reintroduced is a confirmed cause.
• If you cannot safely undo it, show the mechanism with numbers: the modified component reads out of range, the load exceeds the modified part's rating, the timing matches when the change was made. See related: Reading What Changed Since You Left the Job.
• Watch for the reseating trap: if simply disturbing the modified area "fixes" it, you may have reseated a loose connection, not proven the modification is the cause. Confirm the mechanism, not just the coincidence.

Step 5: separate cause from responsibility

Once you have confirmed the modification is the cause, that is a diagnostic fact. Who owns it, and whether you still stand behind your original work, are separate calls. See related: Who's Responsible When Your Work Was Altered, and Stand Behind Old Work That Was Tampered With, Or Not.

Recap

1. Make a dangerous modification safe before anything else.
2. Locate the fault to a component, not an area.
3. Confirm the modification sits in the fault path.
4. Check the change against the rating, spec, or code it should have met.
5. Test the causal link by undoing it or proving the mechanism, then handle responsibility separately.

References

• Trade-standard fault-isolation and causal-verification practice
• Manufacturer documentation and applicable code for component ratings and safety devices
• See related: Reading What Changed Since You Left the Job; Who's Responsible When Your Work Was Altered