Lexus RX (RX 350L, RX450h) 2016-2025 Repair Manual: Short to GND in Immobiliser System Power Source Circuit (B278A)

Lexus RX (RX 350L, RX450h) 2016-2025 Repair Manual / Vehicle Interior / Theft Deterrent / Keyless Entry / Immobiliser System / Short to GND in Immobiliser System Power Source Circuit (B278A)

DESCRIPTION

When there is a short to GND in the power supply for the transponder key amplifier of the engine switch, the certification ECU (smart key ECU assembly) stores this DTC.

DTC No.

Detection Item

DTC Detection Condition

Trouble Area

Note

B278A

Short to GND in Immobiliser System Power Source Circuit

A short to GND in the power supply of the transponder key amplifier of the engine switch (VC5 - VC5) detected. (1 trip detection logic*)
  • Engine switch
  • Certification ECU (smart key ECU assembly)
  • Wire harness or connector

DTC output confirmation operation:

  • With the shift lever in P and the electrical key transmitter sub-assembly held near the engine switch, an engine start operation is performed by pressing and holding the engine switch when the transmitter battery is depleted.
  • *: Only output while a malfunction is present.
Vehicle Condition and Fail-safe Operation when Malfunction Detected

Vehicle Condition when Malfunction Detected

Fail-safe Operation when Malfunction Detected

Engine cannot be started when transmitter battery is depleted by holding transmitter near engine switch and pressing and holding engine switch with shift lever in P

-
Related Data List and Active Test

DTC No.

Data List and Active Test

B278A

-

WIRING DIAGRAM

Refer to B2784.

Click here

CAUTION / NOTICE / HINT

NOTICE:

  • When using the Techstream with the engine switch off, connect the Techstream to the DLC3 and turn a courtesy light switch on and off at intervals of 1.5 seconds or less until communication between the Techstream and the vehicle begins. Then select the vehicle type under manual mode and enter the following menus: Body Electrical / Smart Access. While using the Techstream, periodically turn a courtesy light switch on and off at intervals of 1.5 seconds or less to maintain communication between the Techstream and the vehicle.
  • Before replacing the certification ECU (smart key ECU assembly), refer to Registration.

    Click here

  • After performing repairs, confirm that no DTCs are output by performing "DTC Output Confirmation Operation".

PROCEDURE

1.

CHECK CERTIFICATION ECU (SMART KEY ECU ASSEMBLY)

(a) Using an oscilloscope, check the waveform.

*a

Component with harness connected

(Engine Switch)

-

-

HINT:

Perform this inspection on the engine switch side.

Measurement Condition:

Tester Connection

Condition

Tool Setting

Specified Condition

J14-10 (VC5) - J14-6 (AGND)

Engine switch off, electrical key transmitter sub-assembly not in cabin, within 30 seconds of engine switch pressed

2 V/DIV., 200 ms./DIV.

Pulse generation

OK:

The waveform is similar to that shown in the illustration.

OK

REPLACE ENGINE SWITCH

NG

2.

CHECK HARNESS AND CONNECTOR (CERTIFICATION ECU (SMART KEY ECU ASSEMBLY) - ENGINE SWITCH)

(a) Disconnect the J41 certification ECU (smart key ECU assembly) connector.

(b) Disconnect the J14 engine switch connector.

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

Standard Resistance:

Tester Connection

Condition

Specified Condition

J41-7 (VC5) - J14-10 (VC5)

Always

Below 1 Ω

J41-11 (AGND) - J14-6 (AGND)

Always

Below 1 Ω

J41-7 (VC5) or J14-10 (VC5) - Body ground

Always

10 kΩ or higher

J41-11 (AGND) or J14-6 (AGND) - Body ground

Always

10 kΩ or higher
NG

REPAIR OR REPLACE HARNESS OR CONNECTOR
OK

3.

CHECK CERTIFICATION ECU (SMART KEY ECU ASSEMBLY)

(a) Reconnect the J41 certification ECU (smart key ECU assembly) connector.

(b) Reconnect the J14 engine switch connector.

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

*a

Component with harness connected

(Certification ECU (Smart Key ECU Assembly))

-

-

Standard Voltage:

Tester Connection

Condition

Specified Condition

J41-7 (VC5) - J41-11 (AGND)

Engine switch off, brake pedal not depressed, 30 seconds or more after driver door opened and then closed

Below 1 V
OK

REPLACE ENGINE SWITCH

NG

REPLACE CERTIFICATION ECU (SMART KEY ECU ASSEMBLY)

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