1999 Land Rover Discovery 2 computer replacement

The vehcles ECM (computer) needs to be programmed to the BCU (body control computer)

Engine Immobilized/Mobilized
Passive immobilization prevents vehicle engine from being started unless the authorized key is used. When ignition is switched off, the vehicle remains mobilized for up to 5 minutes. If driver's door is opened after 2.5 minutes, the vehicle remains in a mobilized state for a 30 seconds further. When the timer in the BCU has expired, the vehicle is immobilized. When ignition is turned on, the BCU transmits a coded signal to the ECM. If the coded signal does not correspond to the programmed code in the ECM, the ECM operation is inhibited and the BCU inhibits the starting circuit. For passive immobilization to be activated, the driver's door must be closed, the inertia (fuel cut-off) switch must not be tripped, and the key is not inserted in ignition switch.
Whenever vehicle is immobilized, passive remobilizing of the engine occurs when the vehicle is started. The BCU controls the passive exciter coil (located around the ignition barrel). The passive exciter coil generates a magnetic field, which causes the remote transmitter handset to transmit a remobilizing signal. When the BCU receives a valid signal, it allows the vehicle to be started. If a valid signal from the handset is not received within one minute of the ignition switch being turned to the start position, the BCU stops the passive exciter coil from generating the magnetic field.