When is it worth it?  Traps?  Things to look out for?  Current note is 3.75% 30 year with 317 months left.  Balance of $171k

Depends, if you can do it with no out of pocket it is usually a no brainer.

Keep in mind, unless you go for a shorter term you are going back to 30 years.

Do you plan on moving?  That usually kills any savings.

Basically, find a free on-line program and crunch the #s.

The main thing, watch for fees/costs (of all kinds), that eat in to and offset any savings.

If I go through, it would be to a 15 year.  Not moving.  My credit union advertised a rate of 2.375% which is what got my attention

Get a fee schedule (disclosure), see how much out of pocket and then you can calculate your break even.

Assuming the fees are not too bad, that sounds like a great deal.

15 years at that rate, equity comes off fast.

This^^^

The general rule of thumb given by financial counselors like Dave Ramsey is you need to be saving around 1.25% to make it worth it but that figure is using an assumed refinance cost. If you can get the exact refi cost you can make the calculation yourself. I'm guessing your payment will go up a little. But I didn't actually run the numbers. Back in 2012, 3 yrs into my 30 yr mortgage, I had used extra income from a deployment to shave my 27 yrs remaining down to 14 yrs remaining. Rate on initial mortgage was 5.75%, new rate was 3% on a 10 yr fixed. Taking 14 yrs at 5.75 down to 10 yrs at 3 kept my payment within $10 of the original... I nearly had that mortgage paid off when I found our dream house and started all over again. I do not regret that refinance one bit. But I ran the numbers. I actually could have gotten 2.875% rate but that financial institution wanted $2200 in closing costs. The 3% one only cost $400. The numbers showed it was far better to save the $1800 in closing costs bc the extra .125% on the other loan would have never offset the added closing costs in a 10 yr term.

That's a great rate.  Yours sounds like a no brainer.  Look at the APR as well and the costs, as everybody else is saying.

Im in the middle of doing it right now.  Going from 3.75 to 2.875 payback is 18.5 months for closing cost and will drop the payment 225.00 But Iv run the numbers and if I just keep making the same payment I will cut 36 payments off the back end and 39K in interest.

I could have gone with a 20 or 15 but I can keep making the same payment I have not with a 30 and should something bad happen (job loss) back it down to the lower payment so I stayed with a 30.

I close on my refi on the 15th. Went from 30yr at 4.5 to 15yr at 3.15

Who did you go with? That is about the change I am trying to make...

-shooter

Keep in mind that many "no cost refinances" will simply roll the cost into your new loan balance, so although it costs nothing out of pocket, you're paying it.  About 4 years ago, I was looking to refi.  I had a good deal on a 90% LTV with no PMI but could get better going to 80% LTV.  I had to kick in a little cash, but appreciation  covered most of that.  I found a company on zillow.  I was very clear that I didn't want to pay anything and didn't want anything rolled into the balance of the loan.  They agreed then right before closing said we weren't going to close on time and I had to pay $2300.  I reminded them I got everything back to them within 48 hours, so it was their problem and not mine, but more importantly, I had no skin in the game.  Since this was completely no cost, I didn't even pay for my appraisal.  They already paid for that out of their pocket, and I was going to walk and stick them with the loss.  Somehow they managed to make things happen.

Not looking to hijack, but I’m looking into this currently myself. I plan on selling in 5-6 years.
Currently paying pmi.
4.25%.
I owe 140. My own appraisal based on comps and my updating (no box store special materials), should be around 200. So my new mortgage won’t have pmi. This was my first home I bought when I was 21, 6 years ago.
I’ll be saving $297 per month with a new 30 at 3.375. Break even from closing costs would be under 12 months.

Do it. Look at a 15 yr though, you'd be much further ahead on paying down the principal in 5 yrs when you go to sell, and it's likely your monthly payment will not change from current payment. Another way to say it, you'll have a lot more equity in your property when you go to sell, hence more cash in your pocket. Go for the 15 yr refi.