Why would a new bathroom sink have a sewer smell when first run?

It could be the water smelling from a hot water tank. If the water sits in tank too long it can smell like sewer. Make sure you are running hot water enough to reload hot water tank.

Not hooked up correct

If you hot water tank smells like a sewer you need a new one! !!!

NEW p-traps have nothing to do with this answer either. Either the main stack where the pipes vent out of the roof are clogged, OR the p-trap does not have enough fall in distance from where the pipe comes out of the wall. The object of the p-trap is to hold enough water so the sewer gas cannot come through it and escape in the pipe and backflow so you can smell it.

Kathy may be right. Make sure the bend in the P Trap is lower than the drain pipe out of the wall. It needs to hold water to prevent sewer gas escaping.

Hot water and dishwash liquid and then pour through some draino to clear any gook in the pipes, could be a dead rat. then I'd pour bleach down.

With a new installation. I would disconnect all the drain pipes and make sure everything is connected right. If it only happens when the water is first run you may be getting a puff of sewer gas from the overflow. Make sure that you used the proper down pipe and proper connection. Also make sure the P Trap is lower than the drain. Still say something is not hooked up properly.

If it is new I would suspect sewer gas which means something is not correctly installed or the Ptrap is dry. Check out this article for advice: https://www.angieslist.com/articles/how-dry-p-trap-can-leak-sewer-gas-smells.htm or this tip using water and cooking oil: https://www.familyhandyman.com/plumbing/drain-repair/how-to-seal-basement-water-traps-with-oil/view-all/. Good luck!

The odor can be coming from numerous places. Often the source of the problem is simple, but in rare occasions the problem can be a serious plumbing flaw that is hidden behind a wall.If you have further questions about plumbing feel free to visit their website.

https://www.elizabethdrainservice.com

AC Repair Allen Texas