A backing-up septic system produces the same symptom from causes that differ in cost by two orders of magnitude.
The order to check
1. A blocked effluent filter. Ten minutes, and one of the most common causes. Check this first, always.
2. A tank overdue for pumping, with solids at the outlet.
3. A leaking toilet or a running tap, adding hundreds of gallons a day and hydraulically overloading a field that would otherwise cope. Test with dye in the cistern.
4. A settled distribution box, sending all the flow to one trench.
5. A crushed or blocked pipe between the house and the tank, or the tank and the field.
6. A failed pump or float, on a pumped system.
7. Roots in a pipe or a trench.
8. The drainfield itself.
Only the last is a large number, and it is the one most often assumed.
The diagnostic that separates them
Open the tank. The liquid level says a great deal.
At the outlet invert — normal. The tank is passing effluent and the problem is upstream or in the house.
Above the outlet — effluent is not leaving. Either the outlet is blocked, the filter is clogged, the pipe is blocked, or the field will not accept it.
Below the outlet — the tank is leaking.
Then a hydraulic load test: run water in and see whether the field takes it.
Where repair is worth it
Anything on the list above the field. All of them are inexpensive relative to a field, and all of them are worth ruling out before a replacement is designed.
A tank with failed baffles, which is a cheap fix that protects the field going forward.
A tank that is cracked but the field is sound, which is a smaller job than a field.
Where replacement is the answer
A field that has reached the end of its life, confirmed by a load test rather than assumed.
A system undersized for the house — a four-bedroom house on a three-bedroom system, which is a permitting problem as well as a capacity one.
A cesspool or a system with no tank, which most jurisdictions no longer permit and require to be replaced on failure.
A tank that has collapsed.
Reducing the load as a first response
Where a field is marginal rather than dead:
Fix every leak. Spread laundry across the week. Fit low-flow fixtures. Divert the water softener discharge where permitted. Keep roof and surface water off the field. Stop using a garbage disposal.
That is free, it takes effect immediately, and on a field close to the edge it sometimes restores normal service. It also buys time to plan a replacement rather than being forced into one.
Who to ask
An inspector or designer who is not the contractor who would perform the replacement.
There is no suggestion of dishonesty in the trade generally; it is simply that a diagnosis and a quote from separate parties is how a household knows which of the eight causes above it actually has.
The economics of maintenance
Pumping on schedule, an effluent filter cleaned at each visit, and risers to make both easy, together cost a small fraction of one field replacement over the system's life.
That is the whole argument, and it is why the maintenance items are worth doing even when nothing appears to be wrong — because by the time something appears to be wrong, the damage to the field has already happened.
Getting it in writing
Whatever the diagnosis, ask for the findings rather than the conclusion: the liquid level, the sludge and scum measurements, the condition of the baffles and filter, and what the load test showed.
That set of observations can be taken to a second opinion and compared. A recommendation without them cannot, and for a decision at the scale of a field replacement a second opinion costs a small fraction of the number being quoted.