Quick diagnosis
What this code is telling you
Most likely trigger
Blocked PVC vent exhaust or fresh air intake pipe
Recommended replacement
Furnace Pressure Switch Assembly
Diagnostic path
4 ordered checks
Part fit
Exact model number required
Safety first
Before opening the appliance
Unplug the appliance before removing panels or testing internal components. Do not bypass safety switches or live-test high-voltage, gas, refrigerant, or combustion components unless you are qualified to do so.
Read the full safety disclaimerRoot cause
Why the control reported this error
The furnace control locks out or reports this fault when the expected ignition, airflow, pressure, flame, or limit sequence is not proven. Check blocked PVC vent exhaust or fresh air intake pipe first, then verify cracked or disconnected pressure switch silicone tubing and failed inducer blower assembly before replacing furnace Pressure Switch Assembly.
Symptoms
What you may notice
- ✓Furnace attempts startup but fails to ignite
- ✓Draft inducer motor hums or spins continuously
- ✓Status LED flashes 3 short, 1 long
Possible causes
Check the most likely causes first
- 1
Blocked PVC vent exhaust or fresh air intake pipe
Start here when it matches your symptoms.
- 2
Cracked or disconnected pressure switch silicone tubing
- 3
Failed inducer blower assembly
- 4
Defective pressure switch
Diagnostic sequence
Work through these checks in order
Stop when you find the fault. Do not perform a step that requires skills or live testing you are not qualified to do.
- 1
Check 1
Check exterior PVC flue termination pipes for bird nests, ice, or debris.
- 2
Check 2
Clear condensate drain lines and trap of standing water.
- 3
Check 3
Inspect clear pressure tube for cracks, moisture drops, or loose ports.
- 4
Check 4
Disconnect electrical power before opening panels. Replace draft inducer pressure switch.
Replacement part reference
Furnace Pressure Switch Assembly
Exact OEM part number
Model number required
Fitment note
The exact OEM part number varies by model and production revision. Verify the complete model number before ordering.
Verify the appliance model before ordering.
Manufacturers can use different parts under the same error code across model families and production revisions.
Where do I find my model number? →FixCodeDB may earn a commission from qualifying purchases made through affiliate links.
Common questions
Quick answers for this fault
What should I check first for 31?
Start with blocked pvc vent exhaust or fresh air intake pipe. If that checks out, continue through the diagnostic sequence above rather than replacing a part based on the code alone.
Does 31 always mean Furnace Pressure Switch Assembly is bad?
No. An error code identifies the condition the control detected, not a guaranteed failed part. Confirm the likely causes and wiring first. The exact OEM part number varies by model and production revision. Verify the complete model number before ordering.
When should I stop troubleshooting 31 myself?
Stop if the next step requires live electrical testing, sealed-system work, gas or combustion service, or any procedure outside your training. Unplug the appliance before removing panels or testing internal components. Do not bypass safety switches or live-test high-voltage, gas, refrigerant, or combustion components unless you are qualified to do so.
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