DTC P1789-87: Lost Communication with Gear Shift Control Module "A" from "B" Missing Message [11/2021 - ]: Procedure

WARNING: This page is about a different car, the 2025 Lexus NX 350h, 2024 Lexus NX 350h, 2023 Lexus NX 350h, and 2022 Lexus NX 350h. However, it is still accessible from the selected car via links, so may be relevant.

CHECK HARNESS AND CONNECTOR (SHIFT CONTROL ECU - SHIFT ACTUATOR ECU)

Pre-procedure1

Disconnect the D39 transmission floor shift assembly (shift control ECU) connector.
Disconnect the B78 shift control actuator assembly (shift actuator ECU) connector.
Procedure1

Measure the resistance according to the value(s) in the table below.

Standard Resistance

Tester Connection	Condition	Specified Condition
D39-24 (CA4H) - B78-4 (CA4H)	Always	Below 1 Ω
D39-10 (CA4L) - B78-3 (CA4L)	Always	Below 1 Ω
D39-24 (CA4H) or B78-4 (CA4H) - Body ground and other terminals	Always	10 kΩ or higher
D39-10 (CA4L) or B78-3 (CA4L) - Body ground and other terminals	Always	10 kΩ or higher

Result

Proceed to
OK
NG

Post-procedure1

Connect the shift control actuator assembly (shift actuator ECU) connector.
Connect the transmission floor shift assembly (shift control ECU) connector.

Result:

REPAIR OR REPLACE HARNESS OR CONNECTOR

Result:

See step 2

CHECK TRANSMISSION FLOOR SHIFT ASSEMBLY (SHIFT CONTROL ECU)

Pre-procedure1

Disconnect the B78 shift control actuator assembly (shift actuator ECU) connector.
Procedure1

Measure the resistance according to the value(s) in the table below.

Standard Resistance

Tester Connection	Condition	Specified Condition
B78-4 (CA4H) - B78-3 (CA4L)	Ignition switch off	108 to 132 Ω

Result

Proceed to
OK
NG

Post-procedure1

Connect the shift control actuator assembly (shift actuator ECU) connector.

Result:

REPLACE SHIFT CONTROL ACTUATOR ASSEMBLY (SHIFT ACTUATOR ECU). Refer to REMOVAL [11/2021 - ]

Result:

REPLACE TRANSMISSION FLOOR SHIFT ASSEMBLY (SHIFT CONTROL ECU). Refer to REMOVAL [11/2021 - ]