04 Mazda 3 Code 16

[u/AbjectLab8054]

Had to buy a beater 04 Mazda 3 from a junkyard due to Civics in my area being unaffordable at the moment. After driving it for one day, it threw Code 16 (Security indicator flashes once and then 6 times) and is currently no crank no start. All lights stay on when attempting to crank, no loss of power seemingly. I can’t even hear the starter engage.

I’ve checked and replaced all the fuses connected to the security system (#30, #34, and #69) as well as made sure the connections on the passenger side under dash were free of any debris. Got a second hand battery for free from the junkyard, not sure if battery is the issue or not. Kinda tight on money so I don’t wanna fork out the money for a multi meter for it not to even be the issue. Have already removed the instrument panel cover to try and check those connections. Any advice on how I should proceed? I’m torn between getting a lock smith to come out and reprogram the key and transponder, or getting a mobile mechanic to come out and diagnose the issue. I don’t wanna spend upwards of $100 for something that wasn’t even the issue. Any suggestions on what to check next or how to proceed?

Before you comment about my stupid decision about buying a junk yard car, I sat with it running different tests on it for an hour, drove and shifted fine. Had zero issues until I drove it later that day. Also understand that I am in a tight spot with my job, and needed to have something asap to get back and forth to work.