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Free to use workshop manual for your 2015+ Mk4 'ND' Mazda MX-5

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DTC C0040:23 [START STOP UNIT]

Description	Brake switch (No.1 signal) circuit malfunction
Detection condition	The start stop unit detected the following condition 5 times continuously with the ignition switched ON (engine off or on). The brake switch (No.1 signal) is OFF for a continuous 3 s or more even though the brake switch (No.2 signal) changes.
Fail-safe	Determined by brake switch (No.2 signal) with the brake pedal depressed.
Possible cause	Brake switch connector or terminal malfunction Brake switch (No.1 signal) malfunction Brake switch (No.1 signal) power supply circuit malfunction Short to ground in wiring harness between MAIN 200 A fuse and brake switch terminal A STOP 10 A fuse malfunction MAIN 200 A fuse malfunction Open circuit in wiring harness between battery positive terminal and brake switch terminal A Start stop unit connector or terminal malfunction Short to ground in wiring harness between brake switch terminal D and start stop unit terminal 1C Open circuit in wiring harness between brake switch terminal D and start stop unit terminal 1C Start stop unit malfunction

Diagnostic Procedure

Step	Inspection		Action
1	INSPECT BRAKE SWITCH CONNECTOR CONDITION Switch the ignition off. Disconnect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Disconnect the brake switch connector. Inspect the connector engagement and connection condition and inspect the terminals for damage, deformation, corrosion, or disconnection. Is the connector normal?	Yes	Go to the next step.
		No	Repair or replace the connector, then go to Step 7.
2	INSPECT BRAKE SWITCH (No.1 SIGNAL) Inspect the brake switch (No.1 signal). (See BRAKE SWITCH INSPECTION.) Is the brake switch (No.1 signal) normal?	Yes	Go to the next step.
		No	Replace the brake switch, then go to Step 7. (See BRAKE PEDAL REMOVAL/INSTALLATION.)
3	INSPECT BRAKE SWITCH (No.1 SIGNAL) POWER SUPPLY CIRCUIT FOR OPEN CIRCUIT OR SHORT TO GROUND Verify that the brake switch connector is disconnected. Connect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Measure the voltage at the brake switch terminal A (wiring harness-side). Is the voltage B+?	Yes	Go to the next step.
		No	Inspect the STOP 10 A fuse and MAIN 200 A fuse. Any fuse is blown: Refer to the wiring diagram and verify whether or not there is a common connector between MAIN 200 A fuse and brake switch terminal A. If there is a common connector: Determine the malfunctioning part by inspecting the common connector and the terminal for corrosion, damage, or pin disconnection, and the common wiring harness for a short to ground. Repair or replace the malfunctioning part. If there is no common connector: Repair or replace the wiring harness which has a short to ground. Replace the fuse. Any fuse is damaged: Replace the fuse. All fuses are normal: Refer to the wiring diagram and verify whether or not there is a common connector between battery positive terminal and brake switch terminal A. If there is a common connector: Determine the malfunctioning part by inspecting the common connector and the terminal for corrosion, damage, or pin disconnection, and the common wiring harness for an open circuit. Repair or replace the malfunctioning part. If there is no common connector: Repair or replace the wiring harness which has an open circuit. Go to Step 7.
4	INSPECT START STOP UNIT CONNECTOR CONDITION Disconnect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Disconnect the start stop unit connector. Inspect the connector engagement and connection condition and inspect the terminals for damage, deformation, corrosion, or disconnection. Is the connector normal?	Yes	Go to the next step.
		No	Repair or replace the connector, then go to Step 7.
5	INSPECT BRAKE SWITCH (No.1 SIGNAL) CIRCUIT FOR SHORT TO GROUND Verify that the brake switch and start stop unit connectors are disconnected. Inspect for continuity between brake switch terminal D (wiring harness-side) and body ground. Is there continuity?	Yes	Refer to the wiring diagram and verify whether or not there is a common connector between brake switch terminal D and start stop unit terminal 1C. If there is a common connector: Determine the malfunctioning part by inspecting the common connector and the terminal for corrosion, damage, or pin disconnection, and the common wiring harness for a short to ground. Repair or replace the malfunctioning part. If there is no common connector: Repair or replace the wiring harness which has a short to ground. Go to Step 7.
		No	Go to the next step.
6	INSPECT BRAKE SWITCH (No.1 SIGNAL) CIRCUIT FOR OPEN CIRCUIT Verify that the brake switch and start stop unit connectors are disconnected. Inspect for continuity between brake switch terminal D (wiring harness-side) and start stop unit terminal 1C (wiring harness-side). Is there continuity?	Yes	Go to the next step.
		No	Refer to the wiring diagram and verify whether or not there is a common connector between brake switch terminal D and start stop unit terminal 1C. If there is a common connector: Determine the malfunctioning part by inspecting the common connector and the terminal for corrosion, damage, or pin disconnection, and the common wiring harness for an open circuit. Repair or replace the malfunctioning part. If there is no common connector: Repair or replace the wiring harness which has an open circuit. Go to the next step.
7	VERIFY THAT REPAIRS HAVE BEEN COMPLETED Always reconnect all disconnected connectors. Connect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Clear the DTC for the start stop unit using the M-MDS. (See CLEARING DTC [START STOP UNIT].) Perform the work of depressing the brake pedal for 3 s or more and then releasing it for 5 times or more with the ignition switched ON (engine off or on). Retrieve the start stop unit DTCs using the M-MDS. (See DTC INSPECTION [START STOP UNIT].) Is the same DTC displayed?	Yes	Repeat the inspection from Step 1. If the malfunction recurs, replace the start stop unit. (See START STOP UNIT REMOVAL/INSTALLATION.) Go to the next step.
		No	Go to the next step.
8	VERIFY IF OTHER DTCs DISPLAYED Are any other DTCs displayed?	Yes	Repair or replace the malfunctioning part according to the applicable DTC troubleshooting. (See DTC TABLE [START STOP UNIT].)
		No	DTC troubleshooting completed.

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