Hunter was on time and did a great job in doing the repairs I needed. I would recommend Smith AC if you need good quality service. Thank you Hunter!
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Diagnosed failed integrated control board on a Lennox SL280DFV furnace. Board showing intermittent fault code E250 (limit circuit open), but limit switch testing normal at 0.1Ω. Oscilloscope revealed unstable 24VAC output and microprocessor timing errors. Replaced IFC board, reflashed firmware to v3.28, and recalibrated pressure switches. Also found damaged thermistor (10kΩ @ 77°F spec, reading 13.8kΩ). After component replacement, system now communicating properly with thermostat and maintaining accurate temperature control throughout all operating stages.
