Septic odor is information — and where you smell it matters more than how bad it is. A diagnosis map, from least to most serious.
Counterintuitively, indoor sewer smell usually isn't the septic tank — it's the plumbing between you and it. The usual suspects: a dried-out P-trap in a rarely used fixture (guest bath, basement floor drain — run water in it; if the smell fades in a day, solved), a failed wax ring under a toilet, or a blocked roof vent — NH winters plug vent stacks with snow and ice, forcing sewer gas back through traps. If every drain smells and the system is also draining slowly, then the tank enters the conversation: an overfull tank pushes gas back up the line. Sewer gas in quantity is both foul and unsafe — persistent whole-house odor deserves a same-week call.
This is the one to take seriously. A leach field doing its job is odorless from the surface — effluent disperses below grade and soil treats it. Persistent odor over the field, especially with soggy ground or lush green stripes, means effluent is reaching the surface: the field is saturated or a line has failed. That's not a nuisance; it's the field announcing end-of-life. See the leach field guide — and get an evaluation before winter makes everything harder.
Faint odor directly at the tank covers after heavy use or in humid weather can be ordinary — tanks vent through the house stack, and covers aren't hermetic. Strong or constant tank-area odor suggests an overfull tank, a damaged cover or riser seal, or a failed inlet baffle. If it's been 3+ years since pumping, the diagnosis and the fix are probably the same visit.
One fixture smells → that fixture's trap or seal. Everything smells indoors → venting first, tank second. Yard smells over the field → evaluation, promptly. Smells at the tank + years since pumping → schedule the pumping and get the answer at the open tank.
Usually a dried P-trap in an unused fixture, a failed toilet wax ring, or a snow-blocked roof vent — plumbing-side causes, not the tank. Run water in unused fixtures first; it solves a surprising share of cases.
Persistent odor over the leach field means effluent is reaching the surface — a saturated or failing field. Unlike indoor smells, this one warrants a prompt professional evaluation.
Sewer gas contains hydrogen sulfide and methane — unpleasant at low levels, genuinely hazardous in concentration. Persistent strong indoor odor deserves same-week attention; never lean into a tank opening to investigate.
If the cause is an overfull tank, yes. If it's a dried trap, blocked vent, or failing field, no — which is why locating the smell before assuming the fix saves money.
Describe where and when — half of odor calls resolve with a question or two before a truck ever rolls.