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

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DTC B13C3:09 [START STOP UNIT]

Description	LF control unit malfunction
Detection condition	When any of the following conditions is met: Start stop unit detects that the LF state signal of the LF control unit is low for 5 s or more. With the communication between the start stop unit and LF control unit being performed normally, there is no response from the LF control unit even though there is the signal transmission request from the start stop unit to the LF control unit, and the LF state signal becomes low.
Fail-safe	Inhibits the door lock/unlock control using the advanced keyless entry system.
Possible cause	LF control unit connector or terminal malfunction Start stop unit connector or terminal malfunction Short to ground in wiring harness between start stop unit terminal 2H and LF control unit terminal G Open circuit in wiring harness between start stop unit terminal 2H and LF control unit terminal G LF control unit malfunction Start stop unit malfunction

Diagnostic Procedure

Step	Inspection		Action
1	INSPECT LF CONTROL UNIT CONNECTOR CONDITION Switch the ignition off. Disconnect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Disconnect the LF control 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 6.
2	INSPECT START STOP UNIT CONNECTOR CONDITION 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 6.
3	INSPECT WIRING HARNESS BETWEEN START STOP UNIT AND LF CONTROL UNIT FOR SHORT TO GROUND Verify that the start stop unit and LF control unit connectors are disconnected. Inspect for continuity between LF control unit terminal G (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 start stop unit terminal 2H and LF control unit terminal G. 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 6.
		No	Go to the next step.
4	INSPECT WIRING HARNESS BETWEEN START STOP UNIT AND LF CONTROL UNIT FOR OPEN CIRCUIT Verify that the start stop unit and LF control unit connectors are disconnected. Inspect for continuity between start stop unit terminal 2H (wiring harness-side) and LF control unit terminal G (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 start stop unit terminal 2H and LF control unit terminal G. 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 6.
5	INSPECT LF CONTROL UNIT Inspect the LF control unit. (See LF CONTROL UNIT INSPECTION.) Is the LF control unit normal?	Yes	Go to the next step.
		No	Replace the LF control unit, then go to the next step. (See LF CONTROL UNIT REMOVAL/INSTALLATION.)
6	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].) Switch the ignition ON (engine off or on) and wait for 5 s or more. 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.
7	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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