If the pipe is inside a cabinet or a crawl space where you cannot easily pour water:
Example: Bathroom sink works but laundry tub doesn't — the frozen section is likely along the laundry branch, often in the wall behind the washer or in a nearby crawlspace. how to unfreeze a drain pipe 2021
This is the least invasive method and should be your first attempt. Do not use boiling water; rapid temperature changes can crack PVC pipes. If the pipe is inside a cabinet or
If you’ve tried gentle heat for 2 hours and the pipe remains frozen: Example: Bathroom sink works but laundry tub doesn't
Emergency plumbers in 2021 are busier than ever (thanks, COVID staffing shortages), but a frozen drain is worth the call before it becomes a flood.
If you can safely access the roof vent pipe, pour a bucket of very hot salt water (1 cup salt per gallon) down the vent. Salt lowers the freezing point of water. Do this only if you’re comfortable on a roof – ice is a serious hazard.