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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22745
Experience:  BEng hons Mech engineering, in auto industry 22 years
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My car key wont work - red light shows on key when you press

Customer Question

My car key won't work - red light shows on key when you press the buttons.
Submitted: 4 years ago.
Category: Mercedes
Expert:  Matt replied 4 years ago.


this sounds like the car is out of synchronisation with the key

try this process

How to Program Mercedes Remote Keys

The transponder chip inside the key must be synchronized to the cars
immobiliser (for starting car) to proceed with these steps.

1. Place transmitter key in ignition lock and turn to position 2

2. Extract transmitter key from ignition lock

3. Within 10 seconds:
press (close) pushbutton, hold and simultaneously press (open)
pushbutton five times one after the other release pushbuttons and then
press one of the two pushbuttons once

It may also be necessary to synchronize a transmitter key if the
battery in the transmitter key has been replaced.



if this doesn't work then I'd suggest try resetting the ECU by disconnecting the battery cables and clamping them together for 10 mins. Remove both battery leads from the battery then use a nut and bolt or G-clamp to hold both battery lead terminals together.

This discharges any capacitors / memory within the ECU and around the car so ensures all memory settings are wiped clear - back to the factory default.

This will reset all the systems on the car and you may need the radio security code to get it going again, and ensure that the keys are in your hand when you re-connect the battery as the car may lock and turn on the alarm

and if that doesn't work then its time to get the dealership involved I'm afraid as onyl they carry the top level diagnostics to re-program keys