P1603 Code - General
TCU Communication Error
OBD-II code P1603 indicates an issue with TCU communication.
There are communications that exist between the engine control module, or ECM, and the transaxle control module, or TCM. This communication is done through a control area network, or CAN. With the communication of the CAN, an independent pin and wiring is needed to receive a signal that includes sensor information from the ECM. The more information there is to communicate, the more wiring that is needed. When it comes to CAN communication, all of the information communicated is done alone control modules like the ECM and TCM using CAN lines. When there is a problem with these lines, the ECM will detect the problem and store a P1603 trouble code; the storing of the trouble code is what causes the SES or check engine light to illuminate on the dash.
Possible causes of trouble code P1603 include:
Faulty transaxle control module (TCM)
Faulty engine control module (ECM)
Harness running from the TCM to the ECM has a short
There is a poor electrical connection between the TCM and ECM
Diagnosing the exact cause of the problem should always begin with a visual inspection that could reveal bad wiring, loose connections, or damaged parts. From there, it is a matter of running down the list to determine what needs to be replaced. In general, a bad ECM will cause a loss of power, poor fuel economy, and stalling engine. While a bad TCM will have more prevalent transaxle issues like erratic shifting.
Borrowed from Toyota Parts