DTC P1609 - Engine Control Module (CPU) Malfunction: Condition
WARNING: This page is about a different car, the 1997 Mazda Millenia. However, it is still accessible from the selected car via links, so may be relevant.
DTC is set when Engine Control Module (ECM) does not read DTCs from output devices. Cause is malfunctioning ECM. Replace ECM and erase any DTCs present. See CLEARING CODES in SELF-DIAGNOSTIC SYSTEM section. Retest vehicle.
DTC is set when ECM's knock control integrated circuit is damaged. Cause is malfunctioning ECM. Replace ECM and erase any DTCs present. See CLEARING CODES in SELF-DIAGNOSTIC SYSTEM section. Retest vehicle.