[Lil bastard]

As Paul alludes, the car, not the key goes into sleep mode after 120 Hrs. (5 days) to preserve the battery, it literally stops listening for the key.

Use the key manually and start the car. This 'wakes up' the receiver in the car's alarm system.

Also, be sure to install a fresh battery to the key to insure you have maximum range.

It is not the 'sensor' in the key. This is commonly referred to as the 'Pill', and you should see it once you have the key head apart for the battery swap - be sure to confirm it's existance. This 'Pill' is really a transponder. When you insert the key into the ignition, the car emits a radio signal of very short range (think 1 or 2 meters) in a frequency programmed into the 'Pill'. This radio energy excites the transponder which returns the energy signal in a slightly altered state. Once the car recognizes this frequency, it allows the starter/ignition system to energize and start the car. This 'Pill' doesn't have to be in the key, it is not powered by the battery - it's power comes from the small radio signal emitted by the car, it just needs to be close enough (given it's limited range) to return the signal to the car. This is why there are several cars now that only require you to have the key in your pocket to start the car. The transponder is set, it is not programmed to the car, rather it is the car which is programmed to the key (transponder).

Cheers!