W203 Ignition key

[Old Merc]

I`ve just bought a 2002 C270 CDi ,one key works perfectly. The spare key will not lock or unlock the doors but it does start the car?
I`ve checked the batteries,both show about 3.2 volts. Done the reset,(hold the lock button while pressing the unlock 5 times etc etc.)
Anything else I should try? I suppose it could be the printed circuit in the key ? does it just pull out ?
Any tips would be appreciated.