What is happening
The grease cap has thickened past the trap’s working volume and the inlet is pushing new water into fat instead of water. Flow drops, the sink slows, then stops. It happens on the busiest night because that is when the most hot, greasy water hits a trap that was already over the line.
Do this now
- Stop the water. Plugs out, dish machine off, no mop buckets down the drain.
- Do not pour anything in. Boiling water re-melts the cap and sends it downstream; additives are banned in Stockton and do the same thing slower.
- Look under the lid. Grease at the outlet baffle means the unit is full — and over the 25% ceiling. A thin cap with a dead sink means the blockage is past the trap, which is a different truck.
- Call and say what you saw. That detail decides pump-out versus jetter.
Why this is more than plumbing here
Operating an interceptor over 25% full is a prohibited act under Chapter 13.40, each day counts separately, and the city can order a cleaning within 24 hours. A backup is the symptom the kitchen sees; the overfull interceptor is the thing the ordinance regulates.
So it does not happen again
Run the calculator with your real tank size and meals per day. If the interval it suggests is shorter than the one you have been keeping, that gap is the whole story. Then a schedule, full contents out every time, and the receipt in the three-year file.