DTC P1602: Immobilizer Unit-Pcm Communication Error: Condition

WARNING: This page is about a different car, the 2002 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when command signal from Powertrain Control Module (PCM) to immobilizer unit is out or range. Possible causes are:

Immobilizer unit malfunction.
Immobilizer system coil malfunction.
Key transponder malfunction.
PCM malfunction.
Open circuit between immobilizer unit and PCM, ground or immobilizer system coil.
Short to ground between immobilizer unit and PCM or immobilizer system coil.