DTC P1601 - Communication Line Error (Automatic Transmission): 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 signal voltage from Transmission Control Module (TCM) to Engine Control Module (ECM) is too high. Possible causes are:

Open or short circuit between terminal No. 2K on TCM and terminal No. 1B on ECM.
Open or short circuit between terminal No. 2N on TCM and terminal No. 1F on ECM.