Happened to me. Check behind your radio. Its this canister type deal with two vacum hoses that connect to it. Make sure both of those hoses are connected. If that doesn't help, you might have a vacum leak.

99 Blk SE Sport 2.5 CD4E Euro Focus SVT Wheels 5 % tint Mesh Grilles w/indiglo ford KKM intake filter FMS 9mm wires Superchip Signal mirrors Fog lamp mod Dingless A6 sidemarkers Duals SVT springs/struts