Quick diagnosis
What this code is telling you
Most likely trigger
Clogged condensate drain hose
Recommended replacement
Differential Pressure Switch
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 clogged condensate drain hose first, then verify obstruction in flue exhaust ducting and faulty inducer fan motor before replacing differential Pressure Switch.
Symptoms
What you may notice
- ✓Inducer motor turns on but igniter fails to glow
- ✓System resets repeatedly
- ✓Diagnostic LED flashes 3 times in succession
Possible causes
Check the most likely causes first
- 1
Clogged condensate drain hose
Start here when it matches your symptoms.
- 2
Obstruction in flue exhaust ducting
- 3
Faulty inducer fan motor
- 4
Defective pressure switch contacts
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
Clean out furnace condensate drain trap and internal tubing.
- 2
Check 2
Verify exterior vent terminal is totally free of blockages.
- 3
Check 3
Disconnect electrical power before opening panels. Check pressure switch port with a pin for corrosion or debris.
- 4
Check 4
Disconnect electrical power before opening panels. Test pressure switch contacts under vacuum; replace switch if failing.
Replacement part reference
Differential Pressure Switch
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 3 Flashes?
Start with clogged condensate drain hose. If that checks out, continue through the diagnostic sequence above rather than replacing a part based on the code alone.
Does 3 Flashes always mean Differential Pressure Switch 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 3 Flashes 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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