Code F2 = Condensate pressure switch / drain blocked. Only on condensing Greentherm and Greenstar models. The unit detected backup pressure in the condensate system.
Diagnostic sequence
1. Condensate pump (if installed)
Check that the pump is cycling. Silent = pump failure. Replace ($50-100).
2. Drain line freeze
Winter freezing in unconditioned spaces. Insulate and add heat trace.
3. Drain line sediment
Mineral buildup over years. Clear or replace line.
4. Trap blockage
The trap itself accumulates sediment. Remove, inspect, clear.
5. Neutralizer media exhausted
If you have a neutralizer, the calcium carbonate is consumed over 1-2 years.
Bottom line
F2 is purely mechanical. Find the blockage and clear it.