How do I know if a jar has sealed?
After jars cool, check each one using the methods in the NCHFP guidance on cooling jars and testing seals. A sealed lid is typically drawn down and does not flex when pressed. Look at every jar, and do not assume that one sound means all are sealed.
Remove the bands only as the guidance directs, wipe the jar and label it. Store jars without the band if your source says so, which also lets you spot a lid that has lost its seal later.
Keep in mind that a lid that seemed fine on the counter can fail overnight. Check again the next day and at the first move to storage, so a late failure is caught before the jar goes on the shelf.
- Test each jar after it has cooled completely
- Look for a lid that is drawn down and firm
- Check again when you move jars to storage
- Label jars with the contents and date
Why do jars fail to seal?
Common causes covered in tested guidance include food residue on the jar rim, a chipped rim, a lid or ring that was used incorrectly, overfilled or underfilled jars, air bubbles left in the food, and handling errors during cooling.
Rapid temperature changes, a jar that was moved or jostled while cooling, and equipment problems can also play a part. Because several causes look alike, go through your process step by step against a current tested source.
- Food on the rim or a damaged rim
- Lids or bands used or tightened incorrectly
- Wrong headspace or trapped air
- Disturbed or rapidly cooled jars
What should I do with a jar that did not seal?
Do not leave it on the shelf. What to do next depends on the food, the method and how soon you found the failure, and the options in tested guidance include refrigerating it for prompt use or following a specific reprocessing procedure, with details that differ by food.
Look up the rule in the NCHFP guidance for your food, or call your county extension office, before you decide. Do not guess, and do not apply a rule from a different food or an older book.
Write down the date and what you did for any failed jar. If several jars from one batch failed, think back over each step in order, because a shared cause such as a rim wiped late or a batch cooled in a draft is more likely than bad luck.
- Do not store an unsealed jar at room temperature
- Check NCHFP guidance for your specific food
- Call your extension office if you are unsure
- Label any jar set aside for prompt use
What if a jar loses its seal in storage?
A jar that was sealed and later shows a loose, bulging or leaking lid may be spoiled. Treat it as a spoilage signal and do not taste the contents. NCHFP has guidance on identifying and handling spoiled canned food, including precautions for opening and disposal.
Check the rest of the batch and your storage area. Heat, freezing, damp and rough handling can all affect seals. If the cause is a storage problem, fixing that protects the other jars.
- Do not taste the contents of a failed jar
- Follow NCHFP handling precautions for disposal
- Check neighboring jars from the same batch
- Look at storage temperature and conditions
Does the equipment matter?
Yes. Use jars and lids made for home canning and follow the maker's directions on lids. NCHFP covers recommended jars and lids, and notes that reusing or mismatching parts can cause failures.
Check jars for chips and cracks before each use. If you use a pressure canner, have your extension office advise on testing the gauge, since offices handle this differently. Do not use a damaged jar even if you only need one more.
- Use only jars and lids made for home canning
- Inspect rims for chips every time
- Follow the lid maker's directions
- Ask your extension office about gauge testing
How do I prevent seal failures?
Follow a current tested recipe exactly, with the headspace, rim cleaning and preparation steps it gives. Skipped or shortened steps are a common source of trouble and cannot be offset by extra effort elsewhere.
Keep a log for each batch. Note the recipe source, any problems and the number of failed seals. A pattern, such as repeated failures with one brand of lid or one jar size, tells you what to change.
- Follow the tested recipe without shortcuts
- Wipe rims and check headspace as directed
- Log failures and look for patterns
- Take a class if you are new to canning
Key takeaways
- Test every jar after cooling and again in storage.
- Most seal failures trace to rims, lids, headspace or handling.
- Never leave an unsealed jar on the shelf.
- Use current NCHFP guidance to decide what to do with a failed jar.
- Keep a batch log to find patterns.
Sources and further reading
Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.