Which 3-way switch to replace?

If it used to work, then one (or both) of the switches has probably failed. But, if it has never worked then the more likely cause of what you describe is the wires reversed on the upstairs switch.

Yep - it used to work. I was just trying to figure if there's an easy way to tell which switch failed by the symptoms. Thanks!

Could be either switch is bad OR the switches are both good and a wire to one of switches is not connected. It's not possible to tell which until you dig in. If any switches are 'back stabbed' try moving the wires to the corresponding screws instead. Note that 3 way switches don't really have on and off positions. IF both switches are the same type/brand AND installed in the same orientation then: Both down = on Both up = on, One up & one down = off.

Kevin

best to replace both switches, lets see they are likely the same age, one has failed the other one will shortly.........

they arent expensive best to replace both with quality ones and forget about it for the rest of your life.

they last forever

Since you say it was a dimmer switch, that usually means there are two different kinds of switches, one master and one slave. The usual rules do not apply. But the procedure is about the same. Replace one, being very careful to use the same slave or master type as the one being replaced and that the wires are replaced exactly as they were. Note: If you buy a pair for the replacement, I would replace both to prevent a possible defective switch from damaging a good replacement.

Good Luck

Generally you wouldn't put two dimmers on the same 3-way circuit. Check to make sure both are at the full brightness, if one dimmed, then it might cause the other to seem to not work properly.

I agree with Joseph, you don't wire two three way dimmers to a circuit, so if it indeed worked properly, you have an electronic master-slave system. If the problem is not a loose or disconnected wire, just replace the two part system with new master and slave