Unfortunately, many times a leachfield failure or impending failure is discovered at time of home sale. This is because the adequacy test requirement imposed by the municipality is designed to allow for a certain longevity beyond the test period. Engineers are required to prove that the system can absorb 150 gallons per day, per bedroom in the dwelling. This is well in excess of the usual usage of 50 gallons per day, per person. A new system will absorb many times the required amount.
Systems expire with age and as the clogging biomat expands.
If you are faced with a leachfield replacement you may also have to correct all the prior installation deficiencies and bring the installation up to code. This is all very expensive. If this is installed, the old leachfield can be allowed to rest and eventually be utilized by simply switching the underground valve. This should be considered a good selling point when you are marketing your home. Another consideration is installation of the leachfield under winter conditions.