An open circuit is a break in the current path. It can result from a blown fuse, disconnected connector, broken wire, failed switch, burned relay contact, or internal component break.
Voltage testing is usually faster than chasing continuity through an assembled vehicle. Start where voltage should be present and move along the circuit until it disappears. The fault lies between the last good point and the first bad point.
What to Look For
Do not confuse a reading with a conclusion. A meter showing 12 volts proves voltage is present at that moment; it does not automatically prove the circuit can deliver enough current under load.
Be careful with unloaded circuits. A meter’s very high input resistance can sometimes display voltage through corrosion or electronic leakage even though the circuit cannot supply meaningful current. A suitable load or voltage-drop test may be needed to prove circuit integrity.
Open circuits are especially common where harnesses flex or where connectors have suffered corrosion or previous repair.
Beginner Diagnostic Checklist
- Verify the exact symptom before touching anything.
- Check battery condition and obvious power/ground connections when relevant.
- Check the correct fuse and connector condition.
Stop and get help if: you cannot identify a safe test point or the vehicle may move unexpectedly.