Failed Root Canal Done 15 Year Ago, Abscess Forming, What Is The Best Action? (photo)

It may depend on how large the lesion shows up as being on the x-ray.  If large, definitely try retreat first, or possibly extraction. 

If small, and no symptoms, then your dentist and you might feel comfortable just following it with x-rays every 6-12 months.  If lesion not getting any larger, and still no symptoms, you might be okay with continuing on with the monitoring protocol.  However, if you elect to "watch and wait", then make sure you immediately seek dental treatment if you begin to get any symptoms of pain or discomfort.

Saying all of that, most dentists will advise that root canal teeth that have infections that show up on the x-ray to see an endodontist (root canal specialist) to retreat or decide to watch and wait for more symptoms.

You can try to have it retreated by an endodontist which is a root canal specialist. However there is no guarantee it will be successful and this treatment will be expensive so while it is the best option to save the tooth, you may just want to have it extracted with the intention if having an implant placed in the future. When you see your dentist they can discuss all the different options with you.

Have the root canal redone.  That is the most economical and practical way to go.................................