[400SEL]

I know this seems silly, but it's good to make sure you have made sure the key does not work. Is the battery the right one? And has it been inserted right? If so, then I guess the key does need to be replaced. I have two remote control keys and the previous owner (he had the car for five years) had replaced the battery in both keys wrong. He thought the keys did not work and he told me that when I bought the car. I did not think it was worth spending the money (someone had told me it was about $100) to get a new key, so I never replaced it. However, I bought new batteries and replaced the existing ones the right way (through trial and error) and both keys now work. Imagine my joy when I realized the remote keys work and I did not have to spend any money (other than for the batteries)!