Won't start, or won't stop? Read the holding logic

Some rungs hold themselves on. When a machine starts and instantly drops out, or won't shut off no matter what, the fault is in the holding logic — read the seal-in or the latch and you'll see exactly what's keeping it on or off.

A faultline101 training module · the skill behind six hands-on scenarios

When a machine starts and instantly drops out, or won't shut off no matter what, the fault is in the holding logic — read the seal-in or the latch and it shows what's keeping it on or off.

The idea

A seal-in rung holds an output on by feeding its own result back as a parallel condition around the start — so a momentary start button keeps the machine running. A latch (OTL/OTU) does it differently: one rung sets the bit, a separate rung clears it, and it stays put in between.

These holding circuits cause two signature complaints. 'It starts, then drops right out' means the seal never held — a stop condition or a permissive in the seal path is breaking it. 'It won't shut off' means a latch got set and nothing is clearing it, or a second rung keeps re-sealing it.

You diagnose both the same way: go online and watch the holding condition. See whether the seal establishes and what breaks it, or find the unlatch rung and read why it never fires. The machine is either failing to remember it's on, or refusing to forget — and the logic shows you which.

Six faults you'll work in this module

You don't learn this by reading it — you learn it by doing it on six different faults, each drilling the same skill until it's instinct:

1. Starts, then drops right out. The seal was fine; a real condition kept opening it.
2. It won't shut off. Fix the stuck sensor and the unlatch finally fires.
3. The seal that never catches. The seal needs every condition in its path true to latch.
4. The machine that remembered. Retentive latches remember through a cycle — add a first-scan reset if that's unsafe.
5. Stop pressed, still running. The stop worked; the logic put it right back on.
6. Cold: this thing won't shut down. Cold machine, holding logic read, the real input found.

The mistakes it kills

• Blaming a start button or contactor when the seal-in path is what's actually breaking
• Looking for a stuck output when a latch simply never got its unlatch condition
• Rewriting good holding logic instead of finding the condition that breaks or holds it

Why it matters: Holding logic is where 'it won't start' and 'it won't stop' both live. Reading a seal-in or a latch turns those maddening intermittents into a specific, findable condition.

More: Follow the rung backward · Get the comic · All troubleshooting guides.