Possibly a CAN BUS issue?
 

So just last night I started getting the service tire monitor system message along with service air bag message (that one will usually go away quickly if it comes on, but not this time) and it showed no reading on any of the wheels. Turned it off and back on again, everything seems normal. Today it is doing it again more often than not, with now the radio not powering on and the same messages in the DIC. I'm pretty sure all of these systems are turned on/off via CAN BUS, but I don't really know much about exactly how that system works or what it does. No electrical changes were made prior to this starting either. It's almost as if the battery saver mode is on with a few errors, however I know for a fact it's not a power availability issue. Just wondering if anyone else has had a similar issue.

Check this link out. Sounds somewhat similar.
https://montecarloforum.com/forum/mo...acement-41117/

For some reason the link didn't work. Just type in BCM (body control module) monte carlo on google and there should be a link to a post on this forum. It should be the first result.

Yeah, I've thought about that. The weird thing is that after you park it and turn it off for a while and let it sit there, it usually works until you turn it off and back on again. I wonder if something has happened to one of the circuit boards in the bcm. I can obtain one relatively cheap. Programming, however...

Might have fixed it. Ever since I swapped out to an aftermarket radio, the air bag light has randomly lit up for no apparent reason, but then usually goes back off. When the OE radio is reinstalled, the light never displays except for the systems check when the car is first started.

I jiggled the integration piece between the car and radio (PAC OS-4) and so far everything seems to be good. I didn't always get the same symptoms or messages on the dash and it was very intermittent. So far though, crossing my fingers.

On another note, also changed the water pump today. Had my girlfriend hold the fuse box out of the way when removing/installing the battery. Easily cut 30 minutes off the labor time. For those of you who have fought with the batteries in these, you know what I'm talking about. Get a helper if you can. I will never go back to battery removal alone lol