tldr: 2015 GLA250 -- after some battery changes and weirdness, the car side receiver doesn't seem to be working to recognize my key so I can't use the remote lock/unlock and push button start. I have to use the metal key to lock and unlock the door and manually turn the ignition on without the button.

I have tried:
- replacing the key battery
- re-pairing the key (putting it in the ignition, holding down the lock key and pressing the unlock 5 times, etc)
- replacing the aux battery

I read that the keyless go module might have died but can't figure out which fuse to check and also how to see if it needs to be replaced. Any other ideas?

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So this all started with my dislike for the auto start/stop... I, like a lot of you didn't like it, but fortunately, one day a few years back, I started getting an aux battery error and the auto start/stop stopped working. I considered replacing the aux battery but I liked that the auto start/stop was disabled, everything else seemed ok and the forums told me that I was fine leaving a somewhat low performing aux battery in place.

Fast-forward to three weeks back and I noticed that on colder mornings, my car would struggle to startup intermittently so last week, I decided to replace the main battery. That fixed my startup issues but seemed to make the system think the aux battery was ok so the auto start/stop was re-enabled, but not functional (it would turn off but wouldn't turn on, so I would manually turn it back on, but it would fully restart, so it wouldn't remember that it was still in drive, etc. and turn back on in neutral, the radio, would turn off/on, etc.). After this happened a few times, one time it stopped at a stop light but would refuse to turn back on with the button and I had to remove the button and use the key to get it to start back up.

Since then, the key wouldn't work to lock/unlock the doors and turn the engine on with the button.

I have tried:
- replacing the key battery
- re-pairing the key (putting it in the ignition, holding down the lock key and pressing the unlock 5 times, etc)
- replacing the aux battery

I read that the keyless go module might have died but can't figure out which fuse to check and also how to see if it needs to be replaced. Any other ideas?

Thanks in advance!

 

I was able to fix it!

If anyone else has this issue, try one of the oldest tricks... disconnecting the positive lead of the main battery, waiting 10-15s and then reconnecting it