HVAC Parts by Symptom — What Is Your System Doing?

Most people do not know the name of the part that failed — they know what the system is doing. Find the symptom, and we will tell you which parts cause it and in what order to check them.

Every part below ships the same business day on orders placed by 2 PM CT. $9.98 flat shipping, free once your parts total reaches $75. If you are not sure, call (888) 762-1334 — we install these for a living and we will tell you what to check before you buy anything.

Furnace runs but never lights

You hear the inducer start, maybe the blower, but no flame and no heat. This is the ignition sequence failing.

• Hot surface igniter — Cracked or worn out — the most common cause by a wide margin
• Flame sensor — Sooted or dead, so the board shuts the gas off a few seconds after lighting
• Gas valve — Board is calling, igniter is glowing, no gas arrives
• Control board — Only after ruling out the three above — a board fault is rarer than it looks

Furnace lights, runs a few seconds, then quits — over and over

Classic short-cycle on flame proving. The burner lights and the board cannot confirm it, so it shuts down and tries again until it locks out.

• Flame sensor — Clean the rod first with fine abrasive cloth; replace if cleaning only lasts days
• Control board — If a clean sensor with good continuity still will not prove flame

Furnace heats, shuts off, cools down, starts again

That is the high limit tripping. Almost always an airflow problem, and the limit is reporting it rather than causing it.

• Air filter — Start here. A restricted filter is the single most common root cause
• High limit switch — Replace if it is open at room temperature or will not reset
• Blower motor — A motor dropping out on thermal overload does the same thing

Nothing happens at all on a call for heat

No inducer, no igniter, no blower. Either nothing is telling the furnace to run, or the first step of the sequence cannot start.

• Draft inducer motor — The sequence starts here — if it will not spin, nothing follows
• Control board — No response to a thermostat call at all
• Pressure switch — Inducer runs, then the furnace gives up waiting for the switch to close

Burning smell, or a tripped switch you had to reset

A flame rollout trip means flame escaped the burner compartment. This is the one to take seriously.

• Flame rollout switch — Replace the switch AFTER finding what tripped it — a blocked heat exchanger, failed inducer or plugged vent
• Draft inducer motor — A weak or failed inducer is a common cause

AC hums but the fan will not spin

The outdoor unit is energised and cannot start. If it spins when you nudge it with a stick, you have your answer.

• Run capacitor — Match the microfarad rating exactly — it is printed on the old one
• Hard start kit — For an older compressor that groans on start but otherwise cools fine

Indoor blower runs, outdoor unit does nothing

The thermostat is calling and the high-voltage side is not switching.

• Contactor — Pitted, burnt or stuck contacts, or a dead 24V coil
• Run capacitor — Check this at the same time; they fail together often enough

Water around the furnace or air handler

Condensate is not getting to the drain. On a high-efficiency furnace this can also lock the unit out.

• Condensate drain kit and treatment — A slime-blocked line is the usual culprit
• Float switch — If the unit is locked out with water in the pan, the switch is doing its job
• Pressure switch — Water in the hose or a clogged trap reads as no draft

Weak airflow, or dust everywhere

Airflow problems are cheap to fix and expensive to ignore — they are upstream of half the failures on this page.

• Air filter — Change every 60 to 90 days. Buy the 6-pack and stop thinking about it
• Blower motor — Failing bearings or a motor dropping out on thermal overload
• UV replacement lamp — On an in-duct system, the lamp keeps lighting long after it stops working

Pilot lights, then goes out when you release the button

A standing-pilot system that will not hold. The safety is dropping out because the pilot signal is too weak.

• Thermocouple — Match the length and connection type to the one you are removing

Not on this list?

Browse all replacement parts or call (888) 762-1334. We would rather point you at the right $30 part than sell you the wrong one.