I go to the car, a 2004 Mercedes C240 4Matic, and use the unlock button on my fob. Works fine.

Put the key fob in the ignition. The car lights up with the odometer showing the miles and the park key on the red dot.

Turn the key to the on position and the dash lights up.

I twist to start it up and... nothing.

The dashboard instrument display stays on. It's almost as if the car is acknowledging the key in the ignition. But not the key actually turning to start up the vehicle.

Any help with this would be most welcome. Thanks, and I hope you have a warm week ahead.

The fact that it's happening with cold would tend to indicate a weak battery, however when you have a weak battery you'll usually get a click or a dimming out of the dash when you attempt to engage the starter. I would start with charging and load testing the battery (since it's easy and free), then check electrical connections at the starter.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.