DTC C1A4B: Stop Light Relay Circuit [12/2018 - 07/2020]: Description

The skid control ECU (brake booster with master cylinder assembly) sends a stop light operation request signal to the stop light relay (stop light switch assembly). If the skid control ECU (brake booster with master cylinder assembly) detects a malfunction in the stop light relay circuit, the driving support ECU assembly will store DTC C1A4B.

DTC No.	Detection Item	DTC Detection Condition	Trouble Area
C1A4B	Stop Light Relay Circuit	Diagnosis condition: Power switch on (IG) and auxiliary battery voltage between 11 and 14 V Malfunction condition: Either of the following conditions is met: When the stop light illumination output (STPO) signal is on, a signal has not been input to the STP terminal for 0.3 seconds or more continuously. When the stop light illumination output (STPO) signal is off, signals input to the STP terminal and STP2 terminal of the skid control ECU (brake booster with master cylinder assembly) differ for 0.3 seconds or more continuously.	Lighting system Skid control ECU (Brake booster with master cylinder assembly) Stop light relay (Stop light switch assembly) Wire harness or connector

DTC No.

Detection Item

DTC Detection Condition

Trouble Area

C1A4B

Stop Light Relay Circuit

Diagnosis condition:

Power switch on (IG) and auxiliary battery voltage between 11 and 14 V

Malfunction condition:
Either of the following conditions is met:

When the stop light illumination output (STPO) signal is on, a signal has not been input to the STP terminal for 0.3 seconds or more continuously.
When the stop light illumination output (STPO) signal is off, signals input to the STP terminal and STP2 terminal of the skid control ECU (brake booster with master cylinder assembly) differ for 0.3 seconds or more continuously.

Lighting system
Skid control ECU (Brake booster with master cylinder assembly)
Stop light relay (Stop light switch assembly)
Wire harness or connector