Quick diagnosis
What this code is telling you
Most likely trigger
Closed supply registers
Recommended replacement
Primary Thermal Limit 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 closed supply registers first, then verify extremely clogged furnace filter and failing indoor blower motor before replacing primary Thermal Limit Switch.
Symptoms
What you may notice
- ✓Furnace will not ignite
- ✓Blower fan runs constantly
- ✓LED blinks 1 short, 3 long flashes
Possible causes
Check the most likely causes first
- 1
Closed supply registers
Start here when it matches your symptoms.
- 2
Extremely clogged furnace filter
- 3
Failing indoor blower motor
- 4
Faulty primary limit 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
Replace furnace filter immediately.
- 2
Check 2
Open all home floor vents and duct dampers.
- 3
Check 3
Reset main power disconnect for 30 seconds.
- 4
Check 4
Replace primary thermal limit switch.
Replacement part reference
Primary Thermal Limit 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 13?
Start with closed supply registers. If that checks out, continue through the diagnostic sequence above rather than replacing a part based on the code alone.
Does 13 always mean Primary Thermal Limit 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 13 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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