Carrier: Reversed Polarity or Poor Ground

What this fault means

Carrier boards need correct line polarity and a solid ground to read flame; a fault here causes intermittent no-heat and flame-sense problems.

Likely causes

• Reversed line/neutral at the furnace
• Poor equipment ground
• Loose neutral connection

What you can safely check

• Replace a dirty air filter — restricted airflow causes a surprising share of furnace faults.
• Confirm registers and the outdoor unit are clear of snow, leaves and debris.
• Cycle power at the switch once and let the system retry.

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