MaxC32

FIXED: Parasitic draw issue. No large power draw = no problem.

TD;DR: car seemingly de-syncs keyfob after an hour, key works as expected for an hour when re-synced (inserted into ignition)

03 C32 4 door, when I go out to the car after about an hour or more, the key will not unlock the car.
Unlock with the physical key, insert fob, everything works for a while. Doesn’t have to be driven or even started. Resync procedure seems to yield the same results.

• No tint to interfere with signal.
• 12V Battery is new, and was replaced the right way.
• Fuse 8 replaced.
• Keyfob battery is new.
• Other key tested to same result.

What might cause the vehicle to forget its key such that merely re-inserting it fixed the issue for a short while? Once temporarily fixed, the range on the key is great and the window roll-down & all the buttons work.

MaxC32

Hey folks, some updates in case anyone else is having this problem. Radio was working, but isn’t any longer. The time before the problem occurs seems to be variable but usually around that 30 minute period.
A “noise” was observed by a friend coming from the antenna amp location when the key was inserted after the car couldn’t recognize it. I am going to investigate this area further and report any findings in the interest of solving this issue for anyone else in the future.

Queensnylady

Having the same problem. No one can figure the problem out. Have 2008 s550 I bought a month ago. No manual or fuse diagrams!!!!

MaxC32

I was fixing the oil cooler gasket leak & let the car sit for 5 days (for the first time since purchase). It wouldn’t start & the battery was completely flat. This made me aware of a parasitic drain condition.

Research brought to my attention that the seat control modules are common causes of this condition. Due to some issues with my front seats not controlling right, I guessed that a module was faulty. After unplugging passenger seat control module from power (3 pin plug), parasitic drain AND key issue were resolved.

For anyone reading who has a persistent keyfob issue, try looking for a parasitic draw on your battery!

MaxC32

Here’s a link to a fuse diagram for the W221 chassis on the website that helped me with the C32. I hope it’s useful for you!