Most likely causes (in order of likelihood)

  1. Sail switch not closing — the #1 cause. Blower needs to spin fast enough to push the sail closed before ignition.
  2. Low DC voltage — blower spins, but not fast enough to trigger the sail switch. Common when running off battery without shore power.
  3. Dirty or obstructed burner orifice — spiders love this orifice and clog it. Classic seasonal issue.
  4. Low propane pressure — regulator out of spec, or LP tank near empty.
  5. Failed ignitor or ignitor board.
  6. Bad thermocouple or flame sensor on older units.

Diagnostic steps (in order, free/cheap before expensive)

  1. Listen carefully. You should hear: blower spool up → click of gas valve → whoosh of ignition. If you hear blower but no click, it's the sail switch or board. If you hear click but no whoosh, it's the gas supply or orifice.
  2. Check DC voltage at the battery. You want 12.4V+ for furnace operation. Below 12V, the blower may run but not fast enough to close the sail switch.
  3. If you can access the furnace, pull the cover and locate the sail switch — a small box with a flat metal vane. With the blower running, push the vane closed manually with a pencil. If the furnace then ignites, your sail switch is failing or the blower is weak.
  4. Inspect the burner orifice and tube for spider webs. Mud daubers and spiders nest here constantly. Use compressed air to clear.
  5. Verify propane flow. Light a stove burner on the same propane system to confirm gas is reaching appliances at proper pressure.
  6. If all of the above check out, the ignitor board has likely failed. Replacement is $80-$200 plus install.

DIY vs. call a tech

All steps 1-5 are DIY. Ignitor board replacement is intermediate DIY for anyone comfortable with low-voltage wiring and willing to follow a service manual.

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