This is a classic troubleshooting scenario! Here's my breakdown of the problem and likely cause(s), along with what I'd do to diagnose it further

Understanding the Problem
Basic 3-Way Circuit

A 3-way circuit uses two 3-way switches to control a light from two locations. The key components are: Power source (hot and neutral) Two 3-way switches Travelers (wires connecting the two switches) A switch leg (wire from one switch to the light) Neutral wire (completing the circuit)

Symptoms

One switch (at Box 1 - the power source) doesn't turn off the light. The other switch (at Box 2 - near the light) acts like a regular single-pole switch. Replacing the travelers with a temporary 12-2 cable fixes the problem. Neutral is good and continuous as the light works.

Most Likely Cause

The most likely cause is a break or high resistance in one of the travelers within the existing 12-3 cable between the two switch boxes. When the temporary 12-2 jumper replaces the travelers, it provides a good, low-resistance path, and the circuit functions correctly. When the original travelers are used, one of the travelers cannot make a good contact.

Why this explains the symptoms



Box 2 working like a single-pole

If one traveler is broken, the switch at Box 2 can essentially complete the circuit when it's in the correct position. Think of it as completing the path directly to the light when that switch is in the ON position and there isn't a traveler providing a path back.

Box 1 not turning off

Since one traveler is broken, the switch at Box 1 never makes a proper connection. This leaves one side of the switch connected to the light, even when the switch is in the "off" position.

Other possible, but less probable causes

Backstab connections are not making good contact. Loose wire nut connections Damaged traveler wire insulation somewhere within the walls.

Diagnostic Steps

Here's a logical approach to confirm the issue:1. Double-Check Wire Connections

The easiest things first.

At both switches

Ensure all wires are firmly connected to the switch terminals (screws or push-in connectors). Pay attention to how the wires were connected to the original switches.

Inside the boxes

Verify that all wire nuts are tight and making solid connections.2.

Continuity Test (Key Step)



Power OFF

Turn off the circuit breaker feeding the circuit.

Verify it's off with a non-contact voltage tester. Disconnect

Disconnect the 12-3 cable from both switches (mark the wires first so you know which traveler is which).

Multimeter

Use a multimeter set to continuity or resistance.

Test Travelers

Check continuity between the same color wire at Box 1 and Box 2. You should have near-zero resistance or a beep on the multimeter. Repeat for the other traveler. If one traveler shows open (no continuity) or high resistance, that's your culprit.3.

Inspect the 12-3 Cable (If Possible)



Visible Damage

Look for any signs of damage to the 12-3 cable along its run (nicks, cuts, kinks, etc.).

Termination Points

Carefully inspect where the 12-3 cable enters the boxes. Look for staples that might have damaged the cable.

Solution

Once you confirm the bad traveler: Replace the 12-3 Cable

This is the most reliable fix. Run a new 12-3 cable between the two switch boxes.

Use the Neutral as a Traveler (Generally Not Recommended, but Possible)

If replacing the cable is exceptionally difficult, and only if allowed by your local electrical code, you might be able to repurpose the white (neutral) wire in the 12-3 as a traveler.

Important

You must re-mark the white wire with black or red electrical tape at both ends to indicate it's being used as a hot conductor, not a neutral.

Caution

This is often against code because the white wire is traditionally used for neutral, and someone unfamiliar with the wiring might assume it's a neutral and create a dangerous situation.

Important Safety Notes



Turn off the power before working on any electrical wiring. If you're not comfortable working with electricity, hire a qualified electrician. Always follow your local electrical codes.* Use proper tools and safety equipment.

Good luck! Let me know if you have more questions. Flag for review