Check Engine Light

[Eagle1000]

Just bought a '91 Crown Vic. A couple days after I started driving, the check engine light came on. Took it thru emissions and it passed everything "except" in the PPM catagory.
Any ideas on what could be causing this and how to correct it would be greatly appreciated. Have limited time to go back thru emission test.

One curious note. We had heavy, cold rain here for the past couple days. I drove the car around yesterday and noticed that the check engine light never came on. (It usually comes on after about 10 minutes of driving).
I mention this, hoping it could be a clue to what the problem is.