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Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3666
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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I used the hard key inside the smart one now my Mercedes

Customer Question

I used the hard key inside the smart one now my Mercedes C300 doesn't want to start no answer and the battery is fully charged
JA: Does the engine turn over? How old is the battery?
Customer: The battery is good but no response I ant start the car
JA: Tell me a bit more about what's going on so he can help you best.
Customer: The lights come on the stero is working the alarm is fine but I can't start the engine
JA: Are you hoping to fix this yourself?
Customer: I hope yes
JA: Is there anything else important you think the Mercedes Mechanic should know about your Mercedes?
Customer: No but my Bluetooth was acting up the last time I drove it
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Mercedes Mechanic about your situation and then connect you two.
Submitted: 4 months ago.
Category: Mercedes
Expert:  Technicaldan replied 4 months ago.

Hi, if you had to use the emergency key blade, can you hold the fob right against the start button as you press it.

With the key fob effectively down, the fob needs to be held against the button so the chip can be read similar to the original style chipped keys.

I hope this helps you ;)

Customer: replied 4 months ago.
I don't have a power button I have the smart key and I used the mechanical key to lock the car while it was running After I got home I turned the engine off the next day my car won't start
Expert:  Technicaldan replied 4 months ago.

Just been doing some reading, I opted out in case anyone could help you sooner, as you're still waiting.

Am I right in thinking, you opened and started the car with the smart key, removed the smart key, locked all the doors apart from the drivers, then used the manual key to lock the doors.

How did you then next unlock the door, to turn the engine back off with the fob or the key, I am assuming you removed the fob and took it with you ?

Try unlocking the car using the manual key and not the fob, also have you tried a battery reset? Disconnect it for around five minutes, the main one.

Customer: replied 4 months ago.
I had the smart key inside while the car is running I slid the mechanical key out and kept the smart key inside got out of the car lock it with the mechanical key Came back open the car again with the mechanical one and drove away When I got home I turned the car off. Lock my car with the smart key like usual (remote) The next day no response at all
Expert:  Technicaldan replied 4 months ago.

No response to the key at all, as in remote locking either?

Are there any messages on the screen, does the electric steering lock make the usual noise?

Why did you leave it running?

Customer: replied 4 months ago.
No the fob is working the key is too but when I try to turn the engine on I got no response it's like a safety feature that needs somethings to reset. Cause my car is in perfect condition The battery is too but this happened when I used the mechanical key is stead of the fob
Expert:  Technicaldan replied 4 months ago.

I'm going to opt out so that someone with more knowledge of this system can help you.

I would advise that if they ask you a question you answer them.

In the mean time try a vehicle battery reset.

Another expert will be along shortly.

Expert:  Lou P. master tech replied 4 months ago.

Hello, looking over your info i can tell you that we either have a failed key or a bad ignition switch, there are also possibilities of a failed relay , we will need to do some electrical testing to rule out issues, how familiar are you with a meter and test light?

as for your scenario, you should not have a problem with leaving the key in the ignition and locking the car with the mechanical key , it will not harm anything so i am confident this is just a coincidence. usually what happens, is the ignition switch fails internally and it does not activate the circuit 15 relay in the front fuse box, so not all fuses get power, so what we need to do first is turn the key on and use a test light and check EVERY fuse in the drivers side fuse box for POWER, also keep in mind we are looking for 2 things, please understand what i am asking, i need you to make sure all fuses have power with the key on and also make sure both sides of the fuse has power to make sure none are blown, so we are doing 2 tests in one, if i am right we will have a lot of fuses that will NOT have power. so let me know how you want to proceed , i am not sure of your mechanical abilities so please let me know and we can get this figured out. Lou P.