LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Lexus >> 2017 >> ES 300h >> Repair and Diagnosis >> Restraints >> Restraints Control Systems >> Pre-Collision System (Diagnostic Codes & Circuit Tests) >> Pre-Collision System >> DTC C1A4B: Stop Light Relay Circuit [07/2015 - ] >> Procedure
April 5, 2026: LEMON Manuals is launched! Read the announcement.

DTC C1A4B: Stop Light Relay Circuit [07/2015 - ]: Procedure

  1. INSPECT ENGINE ROOM JUNCTION BLOCK ASSEMBLY 
    1. Turn the power switch off.
    2. Check the engine room junction block assembly. Refer to ON-VEHICLE INSPECTION [07/2015 - ]

      OK

      The engine room junction block assembly is normal.

    Result:  NG

    NG 

    INSPECT ENGINE ROOM JUNCTION BLOCK ASSEMBLY (RESULT OF SIGNAL READING CHECK) 

    Refer to PROCEDURE - Step 2

    Result:  OK

    OK 

    See step  2

  2. CHECK HARNESS AND CONNECTOR (STPO TERMINAL VOLTAGE) 
    1. Turn the power switch off.
    2. Disconnect the A24 brake booster with master cylinder assembly (skid control ECU) connector.
      GTY614174Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
      TEXT IN ILLUSTRATION

      *a Front view of wire harness connector
      (to Brake Booster with Master Cylinder Assembly (Skid Control ECU))
    3. Measure the voltage according to the value(s) in the table below.

      Standard Voltage

      Tester Connection Condition Specified Condition
      A24-3 (STPO) - Body ground Always 11 to 14 V

    Result:  NG

    NG 

    See step  6

    Result:  OK

    OK 

    See step  3

  3. PERFORM ACTIVE TEST USING TECHSTREAM (STOP LIGHT RELAY) 
    1. Select the Active Test on the Techstream. Refer to DATA LIST / ACTIVE TEST [07/2015 - ]
      ABS/VSC/TRAC

      Tester Display Test Part Control Range Diagnostic Note
      Stop Light Relay Switches stop light on/off ON/OFF Test possible at vehicle speed of 0 km/h (0 mph)
    2. Perform the Active Test of the engine room junction block assembly (stop light relay) using the Techstream.
    3. Select the Data List on the Techstream. Refer to DATA LIST / ACTIVE TEST [07/2015 - ]
      ABS/VSC/TRAC

      Tester Display Measurement Item/Range Normal Condition Diagnostic Note
      Stop Light Relay Output STPO terminal output condition/
      ON or OFF      		 ON: Output
      OFF: Not output      		 -
    4. Check for engine room junction block assembly (stop light relay) operation using the Data List and stop light operation by performing an Active Test.

      Result

      Result Proceed to
      Data List content and stop light operation are abnormal. A
      Data List content is normal but stop lights do not turn on or off. B

    Result:  B

    B 

    See step  5

    Result:  A

    A 

    See step  4

  4. INSPECT BRAKE BOOSTER WITH MASTER CYLINDER ASSEMBLY 
    1. Select the Active Test on the Techstream. Refer to DATA LIST / ACTIVE TEST [07/2015 - ]
      ABS/VSC/TRAC

      Tester Display Test Part Control Range Diagnostic Note
      Stop Light Relay Switches stop light on/off ON/OFF Test possible at vehicle speed of 0 km/h (0 mph)
    2. Measure the voltage according to the value(s) in the table below.

      HINT: 

      Do not disconnect the engine room junction block assembly connector.

      Standard Voltage

      Tester Connection Condition Specified Condition
      1G-3 (ACC) - Body ground Power switch on (IG)
      (Active test is OFF)      		 11 to 14 V
      1G-3 (ACC) - Body ground Power switch on (IG)
      (Active test is ON)      		 Below 1.5 V

    Result:  OK

    OK 

    REPLACE ENGINE ROOM JUNCTION BLOCK ASSEMBLY (STOP LIGHT RELAY) 

    Refer to COMPONENTS [07/2015 - ]

    Result:  NG

    NG 

    REPLACE BRAKE BOOSTER WITH MASTER CYLINDER ASSEMBLY 

    Refer to COMPONENTS [07/2015 - ]

  5. CHECK HARNESS AND CONNECTOR 
    1. Turn the power switch off.
    2. Remove the engine room junction block assembly from the engine room relay block and junction block assembly
    3. Disconnect the A24 brake booster with master cylinder assembly (skid control ECU) connector.
    4. Measure the resistance according to the value(s) in the table below.

      Standard Resistance

      Tester Connection Condition Specified Condition
      1F-11 (STOP) - A24-6 (STP2) Always Below 1 Ω

    Result:  OK

    OK 

    REPLACE BRAKE BOOSTER WITH MASTER CYLINDER ASSEMBLY 

    Refer to COMPONENTS [07/2015 - ]

    Result:  NG

    NG 

    REPAIR OR REPLACE HARNESS OR CONNECTOR 

  6. CHECK HARNESS AND CONNECTOR 
    1. Turn the power switch off.
    2. Remove the engine room junction block assembly from the engine room relay block and junction block assembly
    3. Disconnect the A24 brake booster with master cylinder assembly (skid control ECU) connector.
    4. Measure the resistance according to the value(s) in the table below.

      Standard Resistance

      Tester Connection Condition Specified Condition
      1G-3 (ACC) - A24-3 (STPO) Always Below 1 Ω
      1G-3 (ACC) or A24-3 (STPO) - Body ground Always 10 kΩ or higher

    Result:  OK

    OK 

    REPLACE ENGINE ROOM JUNCTION BLOCK ASSEMBLY (STOP LIGHT RELAY) 

    Refer to COMPONENTS [07/2015 - ]

    Result:  NG

    NG 

    REPAIR OR REPLACE HARNESS OR CONNECTOR 

scientia non olet · About LEMON Manuals