Why doesn't my air conditioner's drip pan drain properly?
First - why is your drip pan filling?

Because the process of cooling air requires the use of a condenser which gets very cold.  Humid air when it contacts cold surfaces causes moisture in the air to condense (think how a glass with cold liquid gets condensation on it).

This water falls off the condenser and into the drip pan.

Second - Why is your drip pan over-flowing? 

Sounds like your drip drain is clogged.  You should try to run a thin snake through the entire drain.  Hopefully the drain pipe is big enough to allow this.



Do NOT put drain cleaner in the pipe as, as you've seen, it will drain into your yard and cause damage to plants.