Quick diagnosis
What this code is telling you
Most likely trigger
Clogged air filter
Recommended replacement
High Limit Temperature 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 air filter first, then verify blower wheel covered in dust or animal hair and blocked return ductwork before replacing high Limit Temperature Switch.
Symptoms
What you may notice
- ✓Blower runs continuously while heat call is active
- ✓Burners will not ignite
- ✓Digital display shows E4
Possible causes
Check the most likely causes first
- 1
Clogged air filter
Start here when it matches your symptoms.
- 2
Blower wheel covered in dust or animal hair
- 3
Blocked return ductwork
- 4
Open limit switch contact
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 air filter immediately with correct size.
- 2
Check 2
Inspect blower squirrel cage wheel and clean fins with wire brush.
- 3
Check 3
Verify all dampers and floor registers are fully open.
- 4
Check 4
Disconnect electrical power before opening panels. Measure limit switch continuity when cool; replace limit switch if open.
Replacement part reference
High Limit Temperature 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 E4?
Start with clogged air filter. If that checks out, continue through the diagnostic sequence above rather than replacing a part based on the code alone.
Does E4 always mean High Limit Temperature 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 E4 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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