Code EA = Combustion abnormality. The flame characteristic during operation falls outside the controller's expected envelope.
Diagnostic sequence
1. Vent restriction
Most common cause. Bird nest, ice, debris in vent termination, undersized vent for unit BTU. Inspect intake and exhaust.
2. Gas pressure
Below minimum 4" WC NG / 10" WC propane during fire = lean combustion.
3. LP / NG conversion mismatch
If a unit was converted between NG and LP without proper conversion kit, combustion characteristic will be wrong.
4. Combustion air
Tight installation room without adequate combustion air.
5. Heat exchanger scale
Heavy scale changes burner dynamics. Descale.
Bottom line
EA is usually a system-level issue. Work through the sequence top-down.