99 Cherokee engine light P1494

Hi all
About 10 days ago, I purchased a 99 Jeep cherokee Sport. Since I purchased it, the "check engine" light has come on twice. Since I am under warranty, I had the dealer check it out the first time and their response was that it was a "sensor" and it was fixed. The second time it came on (this past Weds) I went to Autozone to get a free OBD II reading. The code that came up was P1494 (leak detection pump switch or mechanical fault).

This past weekend, I took a 250 mile trip with the engine light on (b/c the dealer said it should be fine to do so) and I noticed that when my air conditioning was on, some light gray smoke was coming out of the exhaust - sporadically. When the A/C was off, I had no problems.

The only other thing I noticed was that the Jeep chuugged a bit this morning when I started it, but it did start up fine.

One other item that you may need to know is that this past Wednesday (the day before the engine light went on 2nd time), I had the MA state inspection done and it passed. I drove the Jeep home and the next time I started it, the check engine light went on.

Any idea what this all means?

Just bought a 2001 Cherokee Sport with the same problem with the engine light! I wish us both luck!