Hey there, just had my water pump changed because it was leaking, no other problems whatsoever.....leaving garage, 15 min later, temp guage goes to red and warning light comes on. Brought back to garage, tell me fans are turning, pump is working. Technician takes car for a spin, tells me that the temp gauge once again went to red but his scanner shows normal operating temp of enigine.....i'm wondering what can be problem and could it have been caused by mechanic when installing water pump, thanxs?

You have air in the system. You need to "bleed" the air out, and your temp guage should return to normal. You didn't say what engine, but I guess it's a 3.1. Follow the coolant bleed proceedure in a repair manual, and you should be fine.