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MB Paul
MB Paul, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4317
Experience:  Mercedes Benz Qualified Diagnostic Technician
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52 plate Mercedes 220 CDi has developed an alarm fault on locking

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52 plate Mercedes 220 CDi has developed an alarm fault on locking with key fob, after just a few secs that alrm sounds and can only be stopped by opening the car door. Hence car is unlocked and not alarmed, or locked with alarm sounding...not ideal...!

Any ideas how to fix...?

MB Paul :

Hi, there are many possibilities for the alarm false alarming when locking the vehicle & to diagnose the fault the car will need to be checked on STAR diagnostic tool to see what fault codes are stored & to see what circuit has triggered the alarm.

MB Paul :

The most common problem by far is with an internal fault in the alarm siren - this has its own battery backup circuit & is a common failure on these models

MB Paul :

If faulty the siren will need to be replaced & then coded on STAR after installation

MB Paul :

Best regards, Paul

Hi, you have rated my answer as "poor service"
Could you please let me know what you have found poor about my answer for you & if you need any further information then you can always reply to me before rating my answer as poor if you need any further help.
Regards, Paul
Customer: replied 3 years ago.

Hi Paul,


I was after a fix for this problem, and not just to be referred to a Mercedes diagnostic test. I could have taken that route without spending £2.


What would be useful would be to advise if there is a way to turn off the alarm, as at the moment I have had to remove the fuse for the central locking system in order to stop the alarm sounding. I would prefer to have central locking working and disengage alarm, is that possible within the system, either with or without removing fuses.




Hi Jason,

Unfortunately the alarm is integral with the security system in the car so there is no way to bypass the system - the only way to prevent the alarm from arming is to lock the car manually (not using the remote). Even then if the alarm siren is bad it can still sound on its own in severe cases due to a built in battery & tamper circuit.

Although nobody likes to hear about diagnostic tests, with a modern day car with networked electronic systems a diagnostic tool is no longer a luxury of a dealer but an essential tool in working on a motor vehicle to fault find - it would be like not having a basic spanner & socket set for any routine maintenence & expecting to be able to service a car!

Pinpointing a fault like this will require a suitable diagnostic tool to see what is setting the alarm off - otherwise you are met with expensive guess work to solve a problem. Not good advice!

I hope you can understand this & why I have given you the answer I have.

Best regards, Paul
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