Furnace Lockout
The furnace tried, failed and stopped trying.
Furnace Blowing Cold Air: A Diagnostic Order That Finds It Fast
Why a gas furnace blows cold or lukewarm air, in the order to check it: thermostat fan setting, lockouts, flame proving, limits, gas supply, and duct leaks.
Furnace Inducer Motor: Testing, Noises and When It's Really the Pressure Switch
How a furnace draft inducer works, how to test one that won't start, runs noisy or runs but won't prove, and how to tell an inducer problem from a pressure switch or venting problem.
How to Test a Furnace Hot Surface Igniter (and Why They Keep Failing)
How to test a hot surface igniter by watching the sequence, checking voltage and measuring resistance, the difference between silicon carbide and silicon nitride igniters, and what shortens their life.
Furnace Flame Sensor: Microamp Readings, Cleaning and When to Replace
How flame rectification works, how to measure flame sensor microamps, what readings are normal, and how to fix weak flame signal.
Furnace Ignition Lockout: Failed Ignition Troubleshooting Step by Step
Diagnose a furnace that tries to light and locks out by following the ignition sequence: inducer, pressure switch, igniter, gas valve, flame sense.
Furnace Pressure Switch Troubleshooting: Open, Stuck Closed and Dropping Out
Diagnose furnace pressure switch codes by measuring the inducer's draft with a manometer and checking vent, intake, hose, condensate trap and switch.