I'm having problems using keyless in the car, and locking the car. Only way to start the vehicle is by putting the key in the "emergency slot" in the center console. 

I've replaced the battery on both sets of keys.

The Ford-dealer informed me that it's not a quick-fix, and it was possible that a transmitter/receiver in the car was defective.

Has anyone had a similar experience, or have any tips on how to fix this myself?

There's usually a receiver in the roof lining for the smaller Fords.  I'm not sure if it's the same for the S-Max though.

First thing to rule out is that you've fitted a decent battery.

Chinese, Ebay, Poundland etc ones are no good.

If you didn't fit an Energizer or Panasonic do that before you do anything else and see if the problem goes away

Then make sure the main 12v battery is in good condition because keyless is the first thing it turns off to save power

2 hours ago, TomsFocus said:

There's usually a receiver in the roof lining for the smaller Fords.  I'm not sure if it's the same for the S-Max though.

It's not good to say where this transmitter/receiver is located.

I assume the dealers keep this type of information tightly locked in their systems.

 

Maybe it's an idea to start dismantling the roof inside the car, then find this part. Then see if you can find something similar on the internet. But this also has to be programmed against the keys. It's questionable whether it's something you can do on your own.

1 hour ago, DaveT70 said:

First thing to rule out is that you've fitted a decent battery.

Chinese, Ebay, Poundland etc ones are no good.

If you didn't fit an Energizer or Panasonic do that before you do anything else and see if the problem goes away

Then make sure the main 12v battery is in good condition because keyless is the first thing it turns off to save power

It is possible that this problem occurred after the battery ran out. I have since changed to a fully functional battery, but the key situation has not been resolved.

26 minutes ago, Baltos said:

It's not good to say where this transmitter/receiver is located.

I assume the dealers keep this type of information tightly locked in their systems.

 

Maybe it's an idea to start dismantling the roof inside the car, then find this part. Then see if you can find something similar on the internet. But this also has to be programmed against the keys. It's questionable whether it's something you can do on your own.

The receiver doesn't have to be programmed.  It just picks up a signal and relays that to the key module.

1 hour ago, Baltos said:

It is possible that this problem occurred after the battery ran out. I have since changed to a fully functional battery, but the key situation has not been resolved.

Which battery?

6 minutes ago, DaveT70 said:

Which battery?

Both car and key battery.

I still find it remarkable that the key works in the emergency slot, but not as keyless.

I also found it strange that no one else has had this problem, and that there are no videos out there describing potential sources of failure.

2 minutes ago, Baltos said:

I still find it remarkable that the key works in the emergency slot, but not as keyless.

That is not strange at all.  The key has a chip inside it, like a non-keyless key.  When you place it in the emergency slot, the chip gets read.  It does not need a battery or an aerial or any buttons working for the chip system.

Drop driver's window

Turn on ignition but do NOT start engine

Disconnect battery negative cable

Cover negative pole on battery with tape or a cloth so it cannot make contact with cable

Disconnect battery positive cable

If they reach, touch positive and negative cables together and hold fort 5 minutes

If they don't reach connect them together with a jump lead or cable and leave for 5 minutes

Unjoin cables

Connect positive cable

Remove cover and connect negative cable

See if passive entry starts working, if not get a sensor and change

This is providing 12v battery is in good condition and fully charged