The combustion air switch on a Coleman furnace ensures safe ignition by confirming adequate airflow through the combustion chamber. When the Coleman Furnace Combustion Air Switch Failed To Close, the furnace will typically lock out or fail to light. This article explains causes, diagnostic steps, safety considerations, common fixes, and preventive maintenance to restore reliable operation.
| Symptom | Possible Cause | Action |
|---|---|---|
| Switch Fails To Close | Blocked intake or vent | Inspect/clear intake; check vent termination |
| Intermittent Operation | Weak inducer motor or wiring | Test motor, capacitor, and wiring; replace as needed |
| Switch Stays Closed | Shorted sensor or stuck actuator | Replace switch or clean mechanism |
How The Combustion Air Switch Works
The combustion air switch (sometimes called a pressure or flow switch) detects airflow produced by the inducer fan and confirms that combustion air and exhaust pathways are clear. The inducer starts first, creating a small pressure differential that the switch senses through tubing connected to the furnace plenum. When the switch sees sufficient pressure, it closes its electrical contacts, allowing the ignition sequence to proceed.
Key Point: If the switch does not close, the control board will prevent the gas valve from opening to avoid unsafe combustion conditions.
Common Symptoms Of A Switch That Fails To Close
When a Coleman furnace combustion air switch fails to close, the homeowner or technician may notice one or more of the following:
- Furnace attempts to start but locks out with an error code indicating a pressure or air switch fault.
- Inducer motor runs but burners never ignite.
- Intermittent heating—sometimes the furnace lights, other times it does not.
- Hissing or unusual noise near vent or inducer during start-up.
Primary Causes Of Failure
Blocked Combustion Air Intake Or Exhaust
Obstructions such as bird nests, snow, ice, leaves, or debris at the intake or exhaust termination can restrict airflow. This prevents the inducer from creating enough pressure for the switch to actuate. Duct or vent blockages inside the system can also cause the same symptom.
Faulty Or Weak Inducer Motor
If the inducer motor runs slowly or cannot reach rated speed due to worn bearings, a failing capacitor, or electrical issues, airflow will be insufficient. The switch may not see the pressure differential needed to close.
Damaged Or Collapsed Tubing To The Switch
The small rubber or plastic tubes that connect the pressure ports to the switch can crack, kink, or collapse. Any leak or restriction in this tubing changes the pressure signal and may prevent proper switch operation.
Maladjusted, Stuck, Or Defective Switch
The switch itself can fail mechanically or electrically. Contacts can corrode, the internal diaphragm can stiffen, or the actuator may stick, producing a closed or open failure. Manufacturing defects and age-related wear are common contributors.
Incorrect Or Loose Wiring And Control Board Faults
Loose connectors, broken wires, or control board faults can prevent the switch signal from registering even when the switch physically closes. Misconfigured or damaged relay circuits can mimic a failed switch.
Tools And Safety Precautions Before Diagnosis
Before troubleshooting, gather a multimeter, manometer or digital gauge (for pressure measurement), basic hand tools, insulated screwdrivers, needle-nose pliers, and replacement tubing and switch if needed. Always turn off power at the furnace service switch or breaker before probing electrical connections.
Safety Reminder: Working on gas and electrical components can be dangerous. If a technician is not comfortable or certified, contact a licensed HVAC professional. Do not bypass safety switches or operate the furnace with known faults.
Step-By-Step Diagnostic Procedure
1. Observe Error Codes And Inducer Operation
Note any control board error codes or LED indicators that reference the pressure or air switch. Start the furnace and confirm the inducer motor runs and whistles up to speed. Listen for unusual noises and watch for consistent operation through the start sequence.
2. Visual Inspection Of Intake/Exhaust Terminations
Inspect the exterior vent terminations for obstructions, ice, or animal nests. Clear visible debris and ensure the termination caps and screens are intact. For high-efficiency units, verify that condensate drains from the exhaust properly.
3. Check Tubing And Vacuum Connections
Inspect pressure tubing from the inducer housing to the switch for cracks, kinks, or loose fittings. Disconnect the tubing at the switch and blow through to ensure it is open. Replace brittle or damaged tubing with heat-rated replacement tubing.
4. Measure Pressure With A Manometer
Attach a manometer to the pressure port at the switch and start the inducer. Compare measured pressure to the furnace specifications in the service manual. If the pressure is below spec, the inducer or airflow path is the likely cause.
5. Test The Switch Electrically
With power off, measure continuity across the switch terminals. With the inducer running, the switch should change state (open to closed) and continuity should reflect that. If mechanical pressure is present but the switch does not actuate, replace the switch.
6. Inspect Inducer Motor And Capacitor
Test the inducer motor for proper voltage and motor winding continuity. Check the capacitor for correct microfarad rating if the motor seems sluggish. Replace worn motors or failed capacitors to restore adequate airflow.
7. Verify Wiring And Control Board Inputs
Confirm that wiring between the switch and the control board is intact and connectors are secure. Use a multimeter to check the control board input for the switch signal during operation. Faulty board inputs may need professional diagnosis or replacement.
Common Repair Options
Clear Obstructions And Service Venting
Clearing intake and exhaust terminations often resolves pressure switch failures. Trim nearby landscaping, relocate blockages, and replace broken vent components. Ensure proper termination height per local code to avoid reoccurrence.
Replace Pressure Tubing
Using OEM or heat-rated replacement tubing restores correct pressure sensing. Secure tubing with clamps if required and route it to avoid sharp bends or heat exposure.
Replace The Combustion Air Switch
If the switch is mechanically or electrically failed, replacement is straightforward. Match the replacement part number to the Coleman furnace model and ensure identical pressure specifications. After replacement, retest pressure and sequence operation.
Repair Or Replace Inducer Motor/Capacitor
Replacing a weak inducer motor or capacitor will restore rated airflow. Motors may be rebuildable in some cases, but replacement is typically recommended for reliability.
Address Control Board Or Wiring Issues
Repair frayed wiring, secure connectors, and replace the control board when necessary. Ensure proper grounding and correct voltage supply to avoid intermittent failures.
Testing After Repair And Commissioning
After repairs, run the furnace through several complete heat cycles to confirm consistent switch closure and ignition. Re-measure pressure during inducer operation and verify the control board no longer logs pressure-related errors.
Documentation: Record pre- and post-repair pressure readings, part numbers replaced, and any adjustments made for future reference.
Preventive Maintenance Tips
- Schedule annual professional furnace service before the heating season to catch wear and prevent failures.
- Keep vent terminations clear and free of debris; install animal guards if appropriate.
- Inspect pressure tubing during routine maintenance and replace if brittle or discolored.
- Replace inducer motor bearings or capacitors at first sign of wear to avoid insufficient airflow.
- Ensure combustion air openings and returns in the mechanical room meet code to maintain proper draft and pressure signals.
When To Call A Professional
Call a licensed HVAC technician if the issue involves gas valve components, suspected gas leaks, complex control board faults, or if diagnostic steps require disassembly beyond the homeowner’s comfort level. Professionals carry the tools and knowledge to safely test gas pressures, replace control boards, and troubleshoot intermittent electrical faults.
Emergency Signs: Strong smell of gas, repeated lockouts after multiple resets, or visible damage to burner components require immediate professional attention and shutting off the gas supply until serviced.
Replacement Parts And Cost Considerations
Common replacement parts include the combustion air/pressure switch, inducer motor, tubing, and capacitors. Part costs vary by model; a replacement pressure switch may range from modest to moderate cost, while inducer motors and control boards are more expensive.
Labor costs depend on region and complexity. Obtaining multiple quotes and using OEM parts for safety-critical components is recommended. Document model and serial numbers when ordering parts to ensure compatibility.
Useful Troubleshooting Checklist
- Record error codes and symptoms.
- Inspect intake and exhaust terminations for obstructions.
- Check pressure tubing for leaks or damage.
- Measure inducer pressure with a manometer and compare to specs.
- Test switch continuity with inducer running.
- Inspect inducer motor and capacitor performance.
- Verify wiring integrity and control board inputs.
- Replace faulty switch, tubing, or inducer components as needed.
Resources And References
Service manuals, OEM part lists, and technical bulletins from Coleman HVAC provide model-specific pressure settings and wiring diagrams. Industry resources from the Air Conditioning Contractors of America (ACCA) and HVACR technical training materials cover safe practices for testing and replacing pressure switches.
Final Note: Addressing a Coleman Furnace Combustion Air Switch Failed To Close issue requires systematic diagnosis of airflow, pressure sensing, and electrical signaling. Proper repair restores safe furnace operation and reduces the risk of recurring faults.