Main electrical panel beside a subpanel with its cover removed
Generic photo: Gvolk · CC BY-SA 3.0

Walking voltage along a schematic rung (hopscotch method) to find the open switch

On a ladder schematic, a load sits on a rung between the two supply legs, with switches in series. Keep one meter lead on the opposite leg (neutral, L2 or 24 V common) and step the other lead along the rung, point by point. Voltage is present up to the open device and missing after it. Then confirm with a voltage reading across the open part.

Cross-checked ~30 min moderateBrands: All

Before you start

  • This is a live-voltage test: insulated probes, one hand where possible, no jewelry.
  • Clip the reference lead before energizing; do not hold two probes on live points across your body.
  • Gas valve circuits on ranges, dryers and furnaces: you may measure voltage to the valve, but gas valve and burner work is blocked and referred.

Diagnostic steps

Unlock all 8 steps with expected values

Full library + AI assistant + the OnCall Tech app for iPhone & Android. $34.90, one time.

Get lifetime access

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

More tech fundamentals guides

Guide · Fundamentals

Blower runs nonstop and no cooling on a new HVAC system: A2L refrigerant leak sensor

A2L systems carry a refrigerant leak sensor near the indoor coil. When it reports a leak, or when the sensor fails or loses communication, the control stops the compressor and runs the blower to dilute any refrigerant. This guide separates a sensor or wiring fault from a possible real leak; any real leak is referred to an EPA 608 technician.

8 steps ~40 minmoderate
Guide · Fundamentals

Confirming an inverter board's DC bus is discharged before you touch it

Inverter boards store high-voltage DC in large capacitors after the power is off, and a dark LED does not mean the board is safe. Remove power, wait the maker's time, then measure the DC bus at the marked test points until it reads 0 V. Only then unplug loads or handle the board.

7 steps ~20 minmoderate
Guide · Fundamentals

Load will not turn off: shorted triac or welded relay (board vs load)

When a load runs with no command, the board output that feeds it is usually shorted: triacs most often fail shorted and relay contacts can weld closed after inrush. Confirm the output is truly on with the load connected (not a ghost reading), then test the load itself, because a shorted or seized load is what usually kills the output.

8 steps ~40 minmoderate
Walking voltage along a schematic rung (hopscotch method) to find the open switch · OnCall Tech