I have a 2004 gmc envoy and the security light came on what does this mean?

SOURCE: 2004 GMC Envoy has trouble idling

Try the fuel filter. Sounds like the engine might be running rough due to weak fuel pressure. Its cheaper to replace, especially if it's still original from 04. Try running some injector cleaner through a couple of tanks of high octane gas.

SOURCE: battery removal on a 2004 GMC Envoy

The battery is on the right side behind the fuse box. You need a metric set to undo the 3 screws. Theres one that attaches to the body, one to the bar that goes along the top, and theres one on the left side below the vacuum hose and air induction. After you've taken out those three, you should lift the whole thing out(battery and shroud) together. It's hard to take the shroud off before you lift the battery out. After that, you have to pry the shoud off the battery. The two terminals in the back make it hard. Then just go to any auto parts store and they'll give you a new one. 

SOURCE: Speedometer on 2004 GMC Envoy not working. It

There is a recall on the panel Cluster... call your dealer and have it checked out.. mention the recall... www.lemonauto.com

SOURCE: I have a 2004 Gmc Envoy putting out a code of 0128

It means the car has not reached operating temp. The computer wants the engine a certain temp so it can go into "closed loop" where it fine tunes the fuel mix for optimum economy and emissions. If it cannot reach that temp it sets that code. You either have a low coolant level or a stuck open/missing thermostat.

SOURCE: What does reduced engine power light mean

this model has what they call fly by wire.. no accelerator cable. If the computer detects a problem in the thottle system it will not allow the engine to accelerate and will show reduced power and possibly set a check engine light. The dealer usually wants to replace the throttle assembly which is very expensive and usually un nessesary.
I have had a lot of succes with just cleaning the throttle plates real good.