DTC U0100: Lost Communication with ECM / PCM "A" [11/2021 - ]: Procedure

WARNING: This page is about a different car, the 2024 Lexus IS 500, 2023 Lexus IS 500, and 2022 Lexus IS 500. However, it is still accessible from the selected car via links, so may be relevant.

CHECK TERMINAL VOLTAGE (POWER SOURCE OF ECM)

Disconnect the A116 ECM connector.

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

*a	Rear view of wire harness connector (to ECM)

Turn the engine switch on (IG).

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

Standard Voltage

Tester Connection	Condition	Specified Condition
A116-6 (+B2) - Body ground	Engine switch on (IG)	11 to 14 V
A116-7 (+B) - Body ground	Engine switch on (IG)	11 to 14 V

Result

Proceed to
OK
NG

Result:

GO TO ECM POWER SOURCE CIRCUIT (ENGINE CONTROL SYSTEM / SFI SYSTEM). Refer to ECM Power Source Circuit [11/2021 - ]

Result:

See step 2

CHECK HARNESS AND CONNECTOR (ECM - BODY GROUND)

Disconnect the F154 ECM connector.

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

Standard Resistance

Tester Connection	Condition	Specified Condition
F154-1 (E1) - Body ground	Always	Below 1 Ω

Result

Proceed to
OK
NG

Result:

REPAIR OR REPLACE HARNESS OR CONNECTOR (ECM - BODY GROUND)

Result:

See step 3

CHECK HARNESS AND CONNECTOR (ECM - TCM)

Disconnect the A117 ECM connector.
Disconnect the A118 TCM connector.

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

Standard Resistance

Tester Connection	Condition	Specified Condition
A117-9 (CAN+) - A118-21 (CAN+)	Always	Below 1 Ω
A117-10 (CAN-) - A118-13 (CAN-)	Always	Below 1 Ω
A117-9 (CAN+) or A118-21 (CAN+) - Body ground	Always	10 kΩ or higher
A117-10 (CAN-) or A118-13 (CAN-) - Body ground	Always	10 kΩ or higher

Result

Proceed to
OK
NG

Result:

REPAIR OR REPLACE HARNESS OR CONNECTOR

Result:

See step 4

REPLACE TCM

Replace the TCM.

Refer to COMPONENTS [11/2021 - ]

Result

Proceed to
NEXT

Result:

See step 5

CHECK DTC OUTPUT

Clear the DTCs.
Powertrain > ECT > Clear DTCs
Start the engine.

Read the DTCs.

Powertrain > ECT > Trouble Codes

Result

Result	Proceed to
DTCs are not output	A
DTC U0100 is output	B

Result:

PERFORM A/T CODE REGISTRATION. Refer to REGISTRATION [11/2021 - ]

Result:

See step 6

REPLACE ECM

Replace the ECM.

Refer to COMPONENTS [11/2021 - ]

Result

Proceed to
NEXT

Result:

PERFORM RESET MEMORY. Refer to INITIALIZATION [11/2021 - ]