New to Forum and I have a problem.
 

Hi, guys and gals. I am new to this forum and look forward to sharing help and ideas.

The battery in my 1999 C280 was dead for a couple of months and I relaced it today. My security alarm/siren will not shut off. I disconnected the wiring connector and it stopped, but now the 'check engine' light is on. Any ideas???

Is it a stock alarm or aftermarket? I am guessing aftermarket since the factory one just uses the horn.

Factory alarm.
 

It is the stock alarm that is wired into the factory harness. No aftermarket mods to the car. Thanks.

Well, I really don't know. The CEL could be for any reason, you just have to scan for the code. I assume you tried arming then disarming the alarm? I have only set the panic off on mine a couple of times accidentally and only the horn beeped. Have you tried searching? Hopefully someone has an idea.