I realized that I did not pressure can my tomato sauce long enough. The lids sealed.. Can I put them back in the canner like that to finish ? It has been several days
The sealing of the lid means that no new bacteria or micro-organisms can get into the food, but if you didn't complete the full pressure cooking process you may not have killed everything that already existed in the jar. If you realized this the same day that you originally canned, you would be fine to re-process (break the seal, dump everything out, wash/sterilize, and try again); but once a few days have gone past you have to assume that the food in the cans has continually been exposed to whatever still may be in there, and the food is going to spoil. For safety reasons it would be best to dispose of the batch and start fresh.
What you suggest boils down to using what is called "double canning" over here in Europe as fallback, assuming that while you may not have reached the proper > 120 °C, you are still sure you did reach 100 °C for the prescribed time to destroy everything but possibly botulinus spores.
This "double canning" is one of the officially recommended options to reduce botulism risk when canning. The procedure is to boil the cans again in water after > 1 day at room temperature. The reason behind this is that canning at 100 °C will destroy botulinus toxins and kill botulinus bacteria but not its spores. You then give the spores time to become normal bacteria and then cook a 2nd time which will kill them as well (and deactivate possibly developed toxins).
(The third recommended procedure is heating immediately prior to eating so toxins of possibly undetected non-proteolytic Cl. botulinum in otherwise not suspicious cans are destroyed.)
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