DTC P1789-87: Lost Communication with Gear Shift Control Module "A" from "B" Missing Message [10/2022 - ]: Procedure
WARNING: This page is about a different car, the 2025 Lexus RX 450h+, 2025 Lexus RX 350h, 2024 Lexus RX 450h+, 2024 Lexus RX 350h, and 2023 Lexus RX 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 C35 transmission floor shift assembly (shift control ECU) connector.
- Disconnect the B57 shift control actuator assembly (shift actuator ECU) connector.
- Measure the resistance according to the value(s) in the table below.
Standard Resistance
Tester Connection Condition Specified Condition Result C35-24 (CA4H) - B57-4 (CA4H) Always Below 1 Ω Ω C35-10(CA4L) - B57-3(CA4L) Always Below 1 Ω Ω C35-24 (CA4H) or B57-4 (CA4H) - Body ground and other terminals Always 10 kΩ or higher kΩ C35-10(CA4L) or B57-3(CA4L) - Body ground and other terminals Always 10 kΩ or higher kΩ 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:
NG
REPAIR OR REPLACE HARNESS OR CONNECTOR
Result:
OK
See step 2
- CHECK TRANSMISSION FLOOR SHIFT ASSEMBLY (SHIFT CONTROL ECU)
Pre-procedure1
- Disconnect the B57 shift control actuator assembly (shift actuator ECU) connector.
- Measure the resistance according to the value(s) in the table below.
Standard Resistance
Tester Connection Condition Specified Condition Result B57-4 (CA4H) - B57-3 (CA4L) Ignition switch off 108 to 132 Ω Ω Result
Proceed to OK NG
Post-procedure1
- Connect the shift control actuator assembly (shift actuator ECU) connector.
Result:
OK
REPLACE SHIFT CONTROL ACTUATOR ASSEMBLY (SHIFT ACTUATOR ECU). Refer to REMOVAL [10/2022 - 08/2023] , or refer to REMOVAL [08/2023 - ]
Result:
NG
REPLACE TRANSMISSION FLOOR SHIFT ASSEMBLY (SHIFT CONTROL ECU). Refer to REMOVAL [10/2022 - ]