If the AC system has correct freon level, and the AC system pressures are normal (Both Low and High) then the problem is the insufficient air flow through the AC condenser/ Radiator. Check for damaged/ collapsed fins, grimes build up on both radiator and condenser, check for things wedge between the two like newspaper, trash bag etc. Cleaning will help to improve the air flow.

Another cause is to check the functions of both electrical fans, they're both control by relay, temperature sensor/ switch and should operate at both low and high speeds. There was a TSB in relation of the cooling fans operation for your 2010 Lacrosse:

TSB Number: PI-0182A NHTSA Number: 10035985 TSB Date: August 1, 2010 Date Added to File: March 7, 2011

Failing Component:

Engine And Engine Cooling:Cooling System:Fan

Summary:

Buick: intermittent and/or inoperative engine cooling fan operation (inspect and fully seat engine cooling fan jumper to forward lamp harness connector x126).

Use the provided information and contact your dealer for information and scheduling for repair.

I am sure that the fans operation was at fault, the PCM controls them so if the PCM has any codes stored related to the fans it would be good to know. If you jump the terminals at the relays and see if the fans come on individually you could see if the fans themselves are good on both low and high speeds. Be sure to check the fuses first.

Engine compartment fuse box:

http://www.autofuseboxdiagram.com/369-20...

Instrument Panel Fuse Box:

http://www.autofuseboxdiagram.com/718-20...

The fans are available at this site (One of many) and what the fans look like:

http://www.carid.com/2010-buick-lacrosse...

Hope it help. HTH

That can be a clogged codenser or a faulty radiator fan. At idle, the fan cools the condenser. At highway speed ram air cools it. If it isn't cooled pressures rise and the high limit switch can shut down the compressor. A pressure guage would tell the story.

The main problem is the loose nut behind the wheel. Your AC compressor needs to be spinning at a certain speed (RPM) before it will pump the cooling medium (Freon) through your system. Apparently, when your car sits at idle, the engine is not running fast enough to turn the compressor at the desired speed.

fan stays on

While the car is in motion, the A/C works fine, but when I'm in slow traffic or at an idle, the air coming out is not cold. I had the Freon checked and it's full. Any ideas what the problem could be?