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Billy, Trainer/ Master Technician
Category: Car
Satisfied Customers: 4918
Experience:  Trainer 18+ years - master tech. 20+ years, SAE member
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Both key fobs do not work. Fob batteries replaced still no

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Both key fobs do not work. Fob batteries replaced still no results. Used metal key to get in but alarm going off. Not now able to lock the car
JA: Have you double-checked to make sure the positive and negative signs are lined up correctly?
Customer: yes
JA: What is the model/year of your Mercedes?
Customer: SLK 2014
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: Checked car battery voltage but it low from the alarm operating, now11.6v
JA: Is there anything else the Mechanic should know before I connect you? Rest assured that they'll be able to help you.
Customer: Not that I can think of

Hello and Welcome! I'm Billy and I will assist you with your inquiry. I understand that having car problems can be frustrating so please feel free to ask if any further info is needed.
If you get prompted for a Premium service or a phone call, please ignore that, cause I don't think it is needed at this point.
My apologies for the delay, thank you for your patience.

A low battery is very likely the root cause of the problem. The alarm system should take power from the auxiliary battery. I would first charge the battery, then try operating the locks or start the engine.

Customer: replied 12 days ago.
Where is the aux battery?
Customer: replied 12 days ago.
Already on a call, so no

No need for a call. This is an automated offer by the system. I will get you the details on the auxiliary battery. But I think that you need to charge the main battery.

Customer: replied 12 days ago.
Not possible to start the car. key does not work

Yes, that is exactly why, I would start by charging the battery. This is to enable the car to start.

The auxiliary battery is located behind the driver's seat:

It's behind an access cover which can be seen as different and can be removed.

Customer: replied 12 days ago.
Is this in the boot or the bonnet
Customer: replied 12 days ago.
forget my last question, just spotted your reply

Technically it's in the boot. But it's accessible from behind the front seat.

Customer: replied 12 days ago.
i will go and check this battery and charge it up if necessary. Probably later tonight, i have to be somewhere else shortly and im getting a lift. can we pick this up tomorrow?

Yes of course. Kindly let me know how it goes, so we proceed as needed.

Billy and 2 other Car Specialists are ready to help you
Customer: replied 12 days ago.
Hi Billy, thanks for your help. I found a useful website for getting at the aux battery but in the end it was not necessary. I called out the RAC/MB engineer and after several diagnostic checks it turns out the keys had failed. Probably because they had been too close to our microwave oven while on holiday. So, I am going to get new keys and hope that cures the issue. Thanks again. Stan