Furnace won't ignite (HSI or spark)

Furnace / gas

AirCompDX - Field Reference for Air Compressor Techs
All HVAC symptoms
1

Cracked / open hot-surface igniter

high
CheckOhm HSI cold — typical 40–90 Ω (Norton) or 50–200 Ω (mini). Open = bad.
FixReplace HSI; never touch element with bare fingers.
ToolsMultimeter
2

No gas / closed valve

critical
CheckManual valve open; inlet pressure 5–7" WC NG, 11–14" WC LP.
FixOpen valve; call utility if no gas at meter.
SafetySmell gas? Evacuate and call utility.
3

Pressure switch not closing

med
CheckInducer running but PS open; check tubing for water, cracks.
FixClear vent, drain condensate trap, replace PS if inducer draw within spec.
4

Bad control board / flame sensor after ignition

med
CheckIgnites then drops out in <7 sec = flame sensor. µA on sensor <0.5 = clean/replace.
FixClean flame rod with fine Scotch-Brite (not sandpaper); replace if still low.
Safety

Pull the disconnect and lock out before opening HVAC equipment. Discharge dual-run capacitors before handling. Refrigerant recovery and brazing require EPA 608 certification and proper PPE.