OL

OL is the display reading on a multimeter or continuity tester when no electrical path exists between the two test points. The resistance measurement reads infinity, or the meter simply shows the letters OL, depending on the meter model. It tells you the circuit is broken: either the wire is cut, the component has failed open, the connection is loose, or the switch is in the off position. In analog meters, the needle sits at the far right end of the resistance scale. In digital meters, OL stands for overload in the original meter design, though it is now universally read as open line by technicians.

When troubleshooting a power supply, lighting circuit, or motor winding, OL immediately narrows the fault location. If you measure across a switch and see OL when it is supposed to be on, the switch is stuck open or the supply wire to it is broken. If you measure across a component like a heating element or resistor and see OL, that component has burned out or failed. Measuring a fuse that shows OL means the fuse has blown. This reading is your first clue that the problem is a break or open failure, not a short circuit.

Limits of the OL reading

OL cannot tell you exactly where the break is, only that one exists somewhere in the measured path. A meter set to a resistance scale of 20 ohms will show OL if the true resistance is above that range; similarly, a meter set to 200 kilohms will show OL if resistance exceeds 2 megaohms. High-voltage insulation tests use megaohm-rated meters specifically to avoid false OL readings on good insulation. Temperature, humidity, and probe contact condition also affect whether a marginal connection appears as a high resistance reading or as OL.

In automotive and industrial wiring, OL on a continuity check is the target result when testing an insulated wire against ground or chassis: no path between them means the insulation is intact. OL is bad only when continuity is required. Skilled technicians use OL readings as a diagnostic gate, asking next whether the open is intentional (switch off, fuse removed) or accidental (wire damaged, joint corroded, contact lost).