My 190E will not lock from the drivers door. It does work from the passenger side and also from the trunk. Unlocking however works fine from that door. Any help on where to start ?

I can't lock my w140, I don't worry,to old to steal,and I take valuables out of car.Been doing it 9 years

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Sounds like a bad lock switch in the door. At minimum the door card will need to come off to inspect.

No! Removing the door card shouldn’t be necessary. Follow these instructions to release the cylinder assembly for inspection:

https://www.coolcatcorp.com/Mercedes%20Locks/Door%20Lock/190%20Door%20Locks.html