My Mercedes S430 won't open trunk and doors don't lock?

Do you have 2 keys or just one? If you have 2, do you have the same symptoms with both?

The first step is to replace the battery in the key. That will solve most problems.

If that does not solve the problem, hold the key about 6 inches from the driver's door handle and try to unlock it. There's an infrared receiver in the door handle for actuating the windows up or down feature. If that works, the RF receiver is bad. That's about $300 - $500 at a dealer including labor. It's also not unusual for both the RF and infrared receivers to go bad -- I've had it happen twice. That's close to $1,000 for both.

If the doors don't lock when you use the locking button in the car, or don't lock when you put it in gear or start it (depending upon the setting) then there may be a problem with the body computer or the data bus. That's several thousand at a dealer.

Let the keys sit for a while/ use the spare if you can. Sometimes the codes just get mixed up. Also, if you keep trying, the codes might realign.

if you are "DIY" lover, you can try to diagnose the malfunction with a MB car diagnostic tool in advance

if there's a battery in the remote, have it changed.