TwoC400s

My 2015 C400 gives me low Key fob battery warnings no matter what I do. I've checked both the battery that was in the fob (3.0V) and the new battery (3.3V) with a multi-meter. So the batteries are obviously good, what could be causing the car to think they're low?

KingsoverQueens

getting the same thing with both of my fobs, comes back on after about a week after the battery was replaced.

Jack Olsen

Same here -- but intermittently. A quick Google search shows this guy's fix:


There is also a reset procedure for the fob:

1. Put the key in the ignition. Do not start the car.
2. Quickly remove it within 10 seconds and hold down the Lock button.
3. While holding the Lock button, press Unlock five times.
4. Release these buttons and press the Trunk button once. This should synchronize your remote.

Three other possible fixes:

A: Get in car - remove keyless go button - start car with key - turn off car - replace keyless button - takes 2 presses to re-start.

B:

1. Start car with key in the ignition
2. Turn off car and remove key from ignition
3. Start car with the Keyless go button on the transmission shifter
4. Turn off car.
5. Repeat 3 and 4.

C:

1. Set in the car with FOB and close the door.
2. Open FOB and remove the battery
3. Install the new fresh battery in FOB
4. Start the car
5. Turn off car.
6. All done

I'm going to print this out and try one at a time until it (hopefully) fixes the problem.

Jack Olsen

Well, I just tried all of the above, including this new AI-generated fix:

To reset the "Replace Key Battery" message on a Mercedes W205 after replacing the key fob battery, follow these steps:

1. Remove the old battery from the key fob and insert the new battery correctly.
2. Get inside the vehicle with the key fob and close all doors.
3. Press the start/stop button once without starting the engine.
4. Remove the key fob from the vehicle and wait 5-10 minutes outside the vehicle's range.
5. Return to the vehicle, enter with the key fob, and start the engine normally.

I also tried alternating my two fobs, as well as both CR2025 and CR2032 batteries (both lithium 3v, different thicknesses) in each. No difference.

Here's what I think. I believe this is a problem that grows progressively worse, but it's intermittent initially -- so a 'fix' seems to work for some people, initially (it did for me) -- that's a symptom of the intermittent nature of the problem.

I think the real problem is probably a gradual failure of whatever antenna is receiving the signal from the fobs.

So, the key question: Does anyone know where the receiver is for the signal from the fob?

TwoC400s

I tried what's in the video and the remote "reset procedure" today with no luck.

Does anyone know what the frequency of the keyless go transmission is? I believe the unlock and lock signals are on 315mhz, but what signal does the keyless go use?

Jack Olsen

That's a great question.

Ideally, we'd be able to identify the whole process that's taking place from the fob's transmission, to the receiver/antenna (I think it's behind the rear seat), to whichever electronic unit is processing it. I haven't dug up my code reader -- but is there any kind of code thrown for this condition? Do W205s without Keyless Go also have the fob's battery monitored?

As a hail-mary idea, I've got a replacement antenna on the way.

TwoC400s

The antenna inside the car? What is the part #?

Jack Olsen

Behind the rear seat, I believe.

Keyless Entry Antenna - 205-905-30-05

Jack Olsen

Well, I did some experimenting while I wait for the new antenna -- and it appears as though I've solved it. For my car, at least.

I ordered a pair of official Mercedes Parts batteries. It turns out they're "Panasonic Industrial" batteries. Of course, all CR2025 batteries conform to the same specs -- they're lithium manganese dioxide batteries that produce 3 volts (+/- 10%). The Industrial label appears to only mean it's from the Panasonic division that sells batteries to industry in bulk.

But here's the surprise: the Mercedes/Panasonic battery immediately made the message go away. I switched back to my spare fob (Maxell battery), and the message came back.

Could Mercedes be doing something to the batteries? I went out and got some off-the-shelf Panasonic batteries (both CR2025 and CR2032) -- and they also work.

So it's not 'Mercedes,' and it's not 'Industrial.' It's just the Panasonic brand, apparently. My Maxell (Japan) and Energizer (China) batteries produce the error message. But Panasonic (formerly Japan, now Indonesia) do not.

So it appears as though it's just the Panasonic brand that makes the difference.

Some more googling showed that this is also a problem in the BMW community, and Panasonic is what they've discovered there as well.

I'm still suspicious. I can't come up with a reason why one battery would work and another with the same specs would not. But I've gone back and forth with other brands now, and Panasonic is the only one that's working.

Not sure what I'll do with that antenna.

Also weird, and worth noting: the Mercedes Panasonic battery showed 3.23v, Panasonic-only branded was 3.28v, and a random Energizer in my drawer tested at 3.32v. So it's not simply voltage that's triggering the system.

TwoC400s

Well, I can't really come up with a rational explanation for it, but the Panasonic battery solved the issue.

So far I'm at 3 days with no low battery warning.

DCF

Same message 'Replace Key Battery' with both fobs in my SL63 even after replacing the batteries. I though it was a problem with the antenna also but ordered some Panasonic batteries and that fixed both fobs. Go figure....

OldManAndHisCar

I found (in my bikes shifters) that if you clean the "bad taste" coating off of the battery you dont get those issues. On my MTN bike I got the error over and again wtih brand new batteries. Cleaned them off with some very nice stuff (brake parts cleaner) on a rag....no more issues. This was a thing on some of the cycling forums I guess.