After double-checking, it seems to be the Flame Rollout Switch.

The circuit board was giving the 33-error code, which is Limit Switch
or Flame Rollout Switch. According to the wiring diagram, there are 3
sensors on the same circuit:

LS - Limit Switch, overtemp - auto reset
ALS - Auxillary Limit Switch, overtemp - manual reset
FRS - Flame Rollout Switch

According to Note #6, ALS is only on downflow models. On mine, the
returns enter the bottom side of the cabinet, and the hot air goes out
the top. I assume that it is an upflow model, and thus does not have an
ALS switch.

The sensor that is tripping is level with the burners (primary heat
exchanger), and on the left. It is a manual reset switch. The other
sensor that is wired in parallel is about level where I would guess the
secondary heat exchanger is. Since the sensor that is tripping is a
manual reset, it does not appear to be LS. By process of elimination, I
assume the switch that is tripping is FRS - Flame Rollout Switch.

The manual does say to check for adequate combustion air and exhaust as
possible problems, as well as the blower.

I compared the intake and exhaust of both furnaces. The furnace having
problems is the larger of the two. The good (smaller) furnace has a
strong flow in the draw and exhaust, and the exhaust is humid and
odorless. The problem furnace has larger pipes, but the flow is
noticeably weaker (covered up the intake with my hand). The exhaust
also has a slight odor, and gives me a headache pretty quickly. That
same odor is slightly noticeable around the problem furnace (there is a
CO detector in the room that has not gone off). I rodded about 5 feet
into each pipe from the outside (about one half of the length), and I
did not hit anything, and it did not make any difference.

Finally, a new problem is the furnace is starting to drip. It is new
because it is just started dripping down the air filter.

Thanks for your help.