I’m looking for some input from people who have dealt with Keyless-Go issues on the S212.

This is a 2011 E350 4MATIC wagon. The problem has happened twice total since I’ve owned the car. It occurred once shortly after I bought it, then disappeared completely for months, and now it has come back again.

Over the past few days I noticed a very specific pattern. Two mornings in a row, when I went to open the driver door, the radio was already on before I touched anything. The car felt like it was already in Key Position 1, but the battery was not dead on those mornings and the car still unlocked normally.

After noticing that, I removed the Keyless-Go start/stop button and left it out for a day. The next morning the car behaved normally with no radio waking and no battery issue. I then put the start/stop button back in, locked the car overnight, and the following morning the battery was completely dead with no response from keyless entry.

The battery is less than a year old and is an AGM, so while I understand it may be stressed now, it doesn’t seem like the original cause. When I scanned the car, I mostly saw low-voltage and communication-related codes, including a Keyless-Go power supply fault, rear SAM or trunk button related fault, tailgate control module configuration error, and multiple CAN communication errors. Once the car has power again, it drives fine and no issues return.

Based on the behavior, it seems like the car is not going to sleep intermittently and something in the Keyless-Go system is waking it. I’m wondering if this could be related to the tailgate handle or antenna on the wagon, a door handle proximity sensor, or some interaction with the rear SAM.

Has anyone experienced the radio already being on before opening the door with Keyless-Go? On S212 wagons, is the tailgate handle a known cause for this kind of behavior? And is permanently disabling Keyless-Go a reasonable long-term solution, or is it worth tracking down the specific component causing the wake-up?

 

I’ve seen this exact behavior before on S212 wagons, and your observations are actually really good. The radio already being on before you touch the door is a classic sign that the car never fully went to sleep (stuck in terminal 15/15R or being periodically woken).



A few points based on experience:

1. Tailgate handle / rear Keyless-Go antenna is a VERY common cause on wagons
   On the S212, the tailgate handle has both the release switch and the Keyless-Go proximity sensor built into it. Moisture intrusion is common, and when it starts to fail it can intermittently “ping” the system, waking the car every few minutes. Because it’s intermittent, it often doesn’t show a hard fault — just low-voltage and CAN communication errors after the battery drains.
2. Rear SAM involvement makes sense here
   The rear SAM controls:
   • Tailgate handle
   • Rear Keyless-Go antenna
   • Trunk button wake signals
     A flaky input from the tailgate handle can keep the rear SAM awake, which then keeps the rest of the car awake over CAN.
4. Your start/stop button test is VERY telling
   Removing the Keyless-Go start/stop button essentially disables Keyless-Go wake authorization. The fact that the problem completely stopped with the button removed strongly suggests:
   • Keyless-Go system is waking the car
   • Not a general parasitic draw
   • Not just a weak battery
6. Door handles can also do this, but wagons are rear-biased
   Front door handles can cause this, but on sedans it’s more common. On wagons, I’d put money on:
   • Tailgate handle first
   • Rear KG antenna second
   • Then door handles
8. Easy diagnostic step before replacing parts
   Have them unplug the tailgate handle connector (inside the tailgate trim) overnight and see if the car sleeps normally.
   If the problem disappears → that’s your culprit.
   This is way easier and cheaper than shotgunning parts.
9. Disabling Keyless-Go is a totally reasonable solution
   Plenty of owners permanently disable it because:
   • Car works perfectly with the key inserted
   • No more battery drain
   • No security downside
     But if they want to keep it, isolating the bad component is worth it.
11. Radio waking first is not random
    The COMAND/radio is one of the first modules to wake when the car comes out of sleep. Seeing it already on before opening the door is a huge clue that something is waking the CAN bus.

If this were my car, I’d start with:

1. Unplug tailgate handle overnight
2. If still happens, unplug rear KG antenna
3. Then look at door handles

Your logic is solid — this doesn’t read like a battery problem at all.

 

You are dealing with a extensive self-feeding chaos. Low voltage disrupt VIP modules (ESP, EZS, F/R SAM,) then begin acting up and keep chassis sleepless.


Let us try to regain focus on root cause here:

-- Use a battery charger to never expose your chassis to 8.5V

-- Clear all faults the rescan fresh

-- Keep the keyless button inserted in EZS

What are the faults now ??


(100% drained flat:
there a chance your young battery has lost a good deal of capacity).