What does E300 mean?
Supply air temperature exceeded the high limit switch setpoint, causing safety shutoff.
Common causes
- Clogged air filter
- Closed supply registers
- Failed blower motor
- Blower capacitor failure
- Heat exchanger restriction
Diagnostic & testing steps
- 1. Check air filter - replace if dirty
- 2. Verify all supply registers are open
- 3. Check blower motor runs at correct speed
- 4. Test blower capacitor - ±6% of rated value
- 5. Measure temperature rise: should be 45-75°F for most furnaces
Tools required
- Multimeter
- Capacitor tester
- Thermometer
OEM parts & part numbers
Blower Motor, Blower Capacitor, Air Filter, High Limit Switch
Part numbers: 45M57, 89W59, 1A2F2
Safety warning
⚠️ CARBON MONOXIDE RISK: Never bypass the high limit switch. Overheating can crack heat exchanger.
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