How to change the battery in the key fog

limited info generic answer check and test battery -- most likely the key fob itself is failing you can have some one test fob or get another fob[s] and have programmed

Question edited for clarity, full maker name.
Question moved to category, albeit '2005'.

Yes, it the battery goes flat, or is disconnected, the anti-theft and alarm needs to be reset.

https://www.google.com/search?q=2003+sl500+reset+security+system

..

You need a mobile auto electrician with a pro grade scan tool computer.

Question edited for clarity.

SL Fuse Chart, Location, Diagram, Allocation 2002-2012 ...

https://www.mercedesmedic.com > sl-class-r230-fuse-ch...
Model: SL350 SL500 SL55 AMG SL600 SL65 AMG SL550 SL55 AMG. Years: 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012. Chassis: R230. Make: Mercedes-Benz ...
Model: SL350 SL500 SL55 AMG SL600 SL65 ...
Make: Mercedes-Benz

Fuse chart for 2003 SL500 - MBWorld.org Forums

https://mbworld.org > ... > SL-Class (R230)
Oct 9, 2018 - SL-Class (R230) - Fuse chart for 2003 SL500 - Does anyone have a copy of the fuse chart that they can email to me? [email protected] Thanks ...
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SL fuse chart.
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R230 Fuse and Relay Diagrams ' Mercedes-Benz Forum

https://www.benzworld.org > ... > R230 SL-Class
Oct 22, 2017 - After seeing many requests for fuse diagrams, and realizing that the fuse panels ... 2003 Mercedes SL, 1964 Corvette StingRay Sport Coupe, ...

R230 Fuse Layout ' Mercedes-Benz Owners' Forums

https://forums.mercedesclub.org.uk > r230-fuse-layout....
Sept 30, 2016 - http://www.mercedesmedic.com/sl-class-r230-fuse-chart/ ... M6 Cab - prior to that, SL63 AMG, A-M V8 Vantage, SL55, SL500 and many more.
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Fuse Box Diagram Mercedes-Benz

https://fuse-box.info > Home > Mercedes-Benz
Fuse box diagrams (location and assignment of the electrical fuses and relays) ... C30, C32, C50) (W203; 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007). In the links.

cause, my.
fob not fog.
the car came with 2 fobs do both fail
never have just 1 fob.
or you will be stranded, and worse costs big time to fix.
a backup key even for SKIM systems can be had cheap.
i have a no fob key that works (skim ) for $13 on my JK jeep.
1: check all fuses first.
2: check that the battery in car is 100% charged, 12.6vdc rested.
3: plug in any Scan tool, got skim errors, report them. (like bad key)
4; lots more... but this gets most.

pull charged battery for 1hr, (neg lug off ) and put back .
no all is reset. try 1 more time.
and with both keys, this time.

If the key fob has stopped working - then the key fob needs a new battery. SmartKey Battery Replacement

The key could be worn down to a point that it isn't fully moving the tumbler. Could be that the ignition lock has been tampered with in such a way that it will no longer respond to the key. If you have a second key that isn't used as much try it; if it works, it's your first key, if it doesn't you might need a new ignition switch.

new battery in fob doesnt mean it cant be faulty retry with a different new battery and or that it is installed correctly -also sometimes a fob will have to be relearned or flashed and that usually means a visit to dealer or service station thats authorized to do so--and if their was rust and oxidation i would recheck and reclean those components again and test continuity

Hello Haq!
Could it be the key just needs re-coding? maybe you are worrying to much as £177 would be about right as that would need coding etc., Try phoning your local Merc dealer or take the key and V5 to them for coding (Hopefully)
Good Luck!
Paul 'W'
Onyer~EDson(:0)~[><

if you hav a remote control key &yr car wont respond . change key batrrey &contact dealer if still dont work for software