If you have a partially draining drain, it will work until it backs
up(assuming you have a float switch). Most do these days. In 90% of
cases you just need to clear the drain
HVAC tech here, there is no universal fix for this problem, it can be
many things. The code means your nest isn't getting 24v power from the
heater. Could be as simple as a clogged drain that is tripping the float
switch or as bad as a blown transfor...
