Ike wrote:
I noticed a large discolored area around the drain pipe outside the
house for the condensation pan. It looked like rust. I got out the
ladder and climbed up in the attic to inspect the two units. One was
fine and the other has rust inside the bottom of the pan. I cannot
tell if the rust is from the pan (nothing obvious) or if rusty water
is draining into the pan. There was no standing water at the time of
inspection, but then the A/C units had not run since last night.

The drain pipes for both pans connect to the side of the pans, not the
bottom. This allows a small amount of standing water to remain in the
pan until it evaporates away. It seems to me a better design would be
to have the drain on the bottom of the pan at the lowest point so
there would never be any standing water.

I am looking for suggestions of whether this pan should be replaced (6
years old) or if I should just keep an eye on it.

I am not sure I totally understand your situation, but I shall
volunteer this: Most air handler units that have the evaporator coils
in them, and have two pans. One is internal and one is external. The
external one is a safety pan, and water in it indicates the internal one
is leaking. It would require opening the air handler to replace this
internal pan, or observe its condition.

If you should ever replace a condensate pan for an evaporator, spray it
with undercoating spray first and let it dry. The undercoating will
make the pan last much longer than just the zinc coating on the sheet metal.

Ken