An open is a break in the path. No path, no current — and voltage tells you exactly where.
A broken wire, a corroded pin, a bad switch, a blown fuse — all opens. With an open, no current flows, so there are no partial drops to chase like in Modules 00–01. Instead the rule is clean and binary: put your voltmeter to ground and walk the circuit. You read full source voltage on the powered side of the break and 0 V on the other side. The break sits right between your last live reading and your first dead one.
12.6 V → 0 V
the step tells you where
Why full voltage?
With no current, there's no voltage drop anywhere (V = I × R, and I = 0). So the whole powered side sits at battery voltage right up to the break, then nothing beyond it. A meter placed across the open reads full source voltage.
⚑ Voltage present, still dead
An open on the ground side is the sneaky one: the load feed still reads ~12.6 V (through the load), but the circuit won't work because current can't return. Voltage at the component does not mean the component has a complete circuit — always confirm the ground.
02 Move the Break
Put the open anywhere and watch the 12 → 0 step move with it.
…
03 What If…
Reason first, then reveal.
?You read 12 V into a switch and 0 V out of it with the switch closed. What's wrong?
The switch is open internally — the break is inside it. Power reaches the input, nothing leaves the output, so the switch contacts aren't making. Replace the switch (or its connector, if that's where the corrosion is).
?The bulb has 12.6 V at its feed but won't light, and there's no blown fuse. Next check?
Check the ground side. Voltage present + no operation is the classic open ground: the feed reads full voltage through the bulb, but current can't return. Probe from the bulb's ground terminal to a known-good ground — an open there confirms it.
?Why does a voltmeter across an open read full battery voltage, but an ohmmeter across the same open reads OL?
Two different tools telling the same story. The voltmeter (circuit live) sees battery voltage pushing against the gap it can't cross. The ohmmeter (circuit dead) sees infinite resistance across the gap = OL. Both confirm an open — one with the circuit on, one off.
04 Practice
ASE-style. Pick an answer to see why.
05 Recap
An open = a break. No current, so no partial drops — it's all-or-nothing.
Voltmeter to ground: full source voltage before the break, 0 V after.
A meter across the open reads full battery voltage.
Open on the ground side = voltage present at the load but it still won't run.
Ohmmeter (circuit off) confirms an open as OL / infinite.