Furnace won't ignite

The sequence begins but the burners never light, or ignition is attempted and fails.

Heating

high

Diagnosed here

Gas Furnace: No Heat
Troubleshooting
The furnace is calling for heat but the burners never light, or they light and then shut down.

Possible cause

Pressure Switch
Component
Safety switches that prove draft or protect a refrigerant circuit, and how to read what an open switch is telling you.

Background

Gas Piping and Gas Pressure
Concept
Supply and manifold pressure, sizing for input, and safe practice around gas connections.
Flame Sensing and Rectification
Concept
How a furnace proves flame with a single rod, and how to measure and interpret the microamp signal.
Ignition Systems
Concept
Standing pilot, intermittent pilot, hot surface and direct spark ignition, and how each proves flame.

Local Code Notice

HVAC installation, venting, combustion air, ventilation air, gas piping, clearances, termination locations, electrical requirements, permits, and equipment requirements vary by jurisdiction and equipment. Always verify the applicable local codes, adopted code edition, regulations, permits, and manufacturer installation instructions before installation or service.