Alright so I've ran into a few problems with my 2003 Monte Carlo LS 3.4.
I've looked a lot on the internet and cant find another person having all the problems with my car. To get this out there, my security light flashes then says service vehicle soon then shows the battery. I do have my check engine light on air idle pressure sensor (something of that sort) an oxygen sensor needs replaced. My car starts up for me every time as I start up to go to work then back home. When my dad drives the Monte he makes a few stops and sometimes wont start up when he goes to get back in ( he disconnects the negative battery cable for 1 min gets back in and car starts up). When we got the check engine light checked at Auto Zone they also checked the battery and alternator both were good. The security light doesn't always immediately start flashing sometimes it doesn't do it for a few miles. Sometimes the cd player which is factory says locked it usually locks more during the day when its warmer then works at night when its colder but it also still sometimes does it at night. Problem #2 when the security light comes on it automatically turns off my automatic lights but when it happens at night I just pull the dome switch clear out an that works. Problem #3 when I go to shutoff the vehicle as the security light is flashing once I turn the key to off my trunk pops open ( we recently just bypassed that to where it wont pop open as it was trying to pop open while driving). We have replaced the ignition switch and the same things still happen. I've heard some of these issues happen to people but not all combined. The car drives great just these darn electrical problems have me at my wits end. Any help is appreciated for sure. I'm thinking along the lines of a bcm or having to replace the ignition lock cylinder. All fuses are good also.

 

Lots of electrical issues, usually indicates a failing BCM. BCM issues will also give you passlock issues. You might have to talk to a GM Shop (or any shop with a tech-2 scanner) and possibly replace the BCM.

 

The flashing security light could be the passlock chip on the ignition cylinder. Did you replace the cylinder with the switch ? However, like chibi said, there seems to be more to it than that.

 

No, we just changed the ignition switch from all the reading I've done I do believe it probably is the BCM going bad. I'm not sure if the bcm controls this but my A/C is also not working now.

 

Get a pressure gauge on the system and see if it has enough of a charge. It might have a leak, and when it gets low the system prevents it from turning on because the freon also has an oil for the system, and you don't want it running without.

 

Alright wanted to update people on what all happened. I talked with my uncle who knows a mechanic. Told him all the problems I was having and the first thing the mechanic said was make sure you have the cleanest connection between the ground wire an the negative battery cable, well dad made sure that was super clean with no corrosion or anything and now the security light is off an my check engine light is off the only thing now is sometimes my car wont start an then you have to disconnect the negative battery terminal for a few seconds then it works again. So we just got a brand new battery today an I'll update as if it will start all the time. Just make sure you have a clean connection with the ground wire as it is a cheap way to start out with fixing a problem because I was going to go straight to replacing the bcm but did this instead. I hope this helps someone out an saves people money.

 

Glad to see you got the problem semi figured out!

 

Thanks for posting what steps you took and your progress!! Many times someone fixes a problem that was posted and you never know what they did or what the outcome was!!

 

Yeah well I know it was frustrating for me didn't have the money for the bcm or for it to be reprogrammed I was worried that was it instead it was a much cheaper step that worked! My check engine came on tonight but that is for my air idle pressure sensor an o2 sensor I believe no big deal now my only problem is the car is not starting im having to go unhook the battery cable then put it back on then it starts an everything is fine so any help on that would be nice. There is no security light on and everything else works good as long as the car starts.

 

Hi, I have an 01' Monte with the starting issue, not all of the other issues yours seems to have. Did you figure yours out? Here is a something I found and a website I found just read disclaimer thank you.

Place vehicle in park, shut off ignition get out of the car, lock it. unlock get back in insert key to acc, turn lights on and off 3 times then turn ignition off and on to start the engine.