HVAC Technician Academy
Learn how a gas furnace moves from thermostat call to inducer, pressure switch, igniter, gas valve, flame proving, and blower operation — and how to diagnose exactly where the sequence stops.
A typical furnace receives a heat call, starts the inducer, proves draft through the pressure-switch circuit, energizes the ignition system, opens the gas valve, proves flame, and then starts the circulating blower after a programmed delay. If any required safety or proof step fails, the control board may stop the sequence and generate a fault code.
A furnace should not be diagnosed by jumping randomly from component to component.
Instead, determine:
Exact timing and logic vary by furnace design. Always verify the manufacturer sequence for the equipment being serviced.
The thermostat typically sends the heating call through W.
Before diagnosing furnace components, verify:
If the board never receives the call, the furnace cannot begin the heating sequence.
Once the board receives a valid heating demand, it commonly energizes the draft inducer.
The inducer helps establish the required combustion-air and venting conditions.
If the inducer does not start, check:
The pressure switch is part of a safety/proving circuit.
It confirms that the inducer has created the pressure condition expected by the furnace design.
If it does not close, investigate the system causing the pressure condition before replacing the switch.
Possible causes include:
When an energized control circuit is open at the pressure switch, you may measure control voltage across the switch.
Once the switch closes successfully, voltage drop across the closed contacts should normally become very small.
This makes voltage-drop logic useful in furnace diagnostics.
After the pressure-switch circuit is proven, the board begins the ignition sequence.
Depending on furnace design, this may use:
If the igniter does not operate, determine whether the board is supplying the correct voltage before condemning the igniter.
A failed hot-surface igniter may be visibly cracked, but visual inspection alone is not enough.
With power removed, resistance can be tested and compared with manufacturer data where available.
With live diagnostic testing performed safely, verify whether the board is actually supplying the required ignition voltage during the ignition window.
After ignition is established or the igniter has reached the proper stage, the board energizes the gas valve.
If the gas valve does not open, determine:
Gas-pressure testing and combustion work should follow manufacturer specifications and applicable fuel-gas procedures.
Ignition is not enough.
The control board must verify that flame is actually present.
Many furnaces use flame rectification through a flame sensor.
If flame is not proven within the allowed time, the board closes the gas valve.
A common flame-sensor symptom is:
Possible causes include:
Where manufacturer procedures support it, flame signal can be measured in microamps DC.
Compare the measured signal with manufacturer requirements.
Do not ignore abnormal ignition characteristics.
Investigate symptoms such as:
These can involve burner contamination, gas pressure, venting, combustion air, heat exchanger conditions, or other serious faults.
After stable flame is proven, the furnace commonly waits a programmed period before starting the indoor blower.
If the burners operate but the blower does not start, check:
Once the furnace reaches stable operation, measure temperature rise.
Compare:
Compare the result with the furnace nameplate range.
Excessive rise can indicate insufficient airflow or excessive input.
Low rise can point toward excessive airflow, low input, or another performance issue.
A high-limit switch opening should trigger investigation into why furnace temperature became excessive.
Possible causes include:
Do not simply replace or bypass the limit without finding the cause.
When the thermostat is satisfied:
The post-purge and blower-delay sequence vary by equipment.
Technician Case File
Heat call: present.
Inducer: operating.
Igniter: never energizes.
Pressure-switch circuit: remains open.
Technician initially considers replacing the pressure switch.
Further testing finds the furnace condensate drain restricted and the pressure tubing full of water.
Lesson: The pressure switch was reporting a system problem. It was not necessarily the failed component.
Inducer: normal.
Pressure switch: proves.
Igniter: normal.
Gas valve: opens.
Burners: ignite.
Several seconds later: gas valve closes.
Flame signal is weak.
The sequence narrows the diagnosis toward flame proving rather than the inducer, pressure switch, or gas valve.
Burner ignition: normal.
Flame proving: normal.
Blower: operating.
Furnace runs several minutes and burners shut down on high limit.
Temperature rise: above nameplate range.
Static pressure: excessive.
The limit is reacting to an airflow problem. Replacing the limit would not repair the furnace.
1. Confirm thermostat heat call.
2. Verify board/control power.
3. Confirm inducer starts.
4. Verify pressure-switch circuit proves properly.
5. Verify igniter operation.
6. Verify gas-valve command.
7. Confirm smooth burner ignition.
8. Measure/verify flame signal.
9. Verify indoor blower starts.
10. Check temperature rise and airflow.
11. Observe the complete shutdown sequence.
12. Review fault history and confirm reliable operation before leaving.
| Where Sequence Stops | Possible Direction | Next Check |
|---|---|---|
| No inducer | Heat call, board, motor, power | W signal and inducer voltage |
| Inducer runs, no ignition | Pressure-switch/draft issue | Vent, tubing, condensate, draft pressure |
| Igniter operates, no gas | Gas valve/control/gas supply | Valve command and gas supply |
| Burners ignite then shut off | Flame proving | Flame sensor signal and grounding |
| Runs then trips high limit | Airflow/input issue | Temperature rise, static, blower, gas input |
On a furnace, do not ask “What part is bad?” first. Ask “What was supposed to happen next, and what prevented it?”
1. What normally starts after the control board receives a heat call?
2. What does the pressure switch prove?
3. If burners light and then shut off after a few seconds, what diagnostic area becomes especially important?
4. Should a high-limit switch be bypassed to keep the furnace running?
5. What is the best way to diagnose a furnace sequence problem?
1. The draft inducer normally starts early in the sequence.
2. It proves that the required draft/pressure condition exists for safe furnace operation.
3. Flame proving, including flame-sensor signal and burner grounding.
4. No. Diagnose why the limit is opening.
5. Follow the manufacturer sequence and identify the last successful step and first missing step.
Return to the heating diagnostic hub.
Furnace Repair →Review the local furnace service resource.
Heating Not Working →Review common no-heat diagnostic symptoms.
Gas-furnace diagnosis involves fuel gas, combustion products, hot surfaces, moving equipment, live line voltage, ignition systems, and carbon-monoxide risk. Work should be performed by qualified personnel following manufacturer instructions, applicable fuel-gas and electrical procedures, and proper combustion-testing practices. Never bypass limit, rollout, pressure-switch, or other safety devices as a permanent repair.
Inducers, pressure switches, ignition components, gas valves, flame sensors, airflow, and safety controls all operate in sequence. Find the step that failed instead of guessing.
Furnace Diagnostic Service HVAC Technician Academy