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

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DTC U0028:87 [START STOP UNIT]

Description	Communication error with BCM
Detection condition	Start stop unit detects the communication error with BCM 10 times continuously.
Fail-safe	Performs control with the driver's side door and trunk open.
Possible cause	BCM connector or terminal malfunction Start stop unit connector or terminal malfunction Short to ground in wiring harness between start stop unit terminal 1Q and BCM terminal 2J Open circuit in wiring harness between start stop unit terminal 1Q and BCM terminal 2J BCM malfunction Start stop unit malfunction

Diagnostic Procedure

Step	Inspection		Action
1	INSPECT BCM CONNECTOR CONDITION Switch the ignition off. Disconnect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Disconnect the BCM 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 BCM AND START STOP UNIT FOR SHORT TO GROUND Verify that the BCM and start stop unit connectors are disconnected. Inspect for continuity between BCM terminal 2J (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 1Q and BCM terminal 2J. 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 BCM AND START STOP UNIT FOR OPEN CIRCUIT Verify that the BCM and start stop unit connectors are disconnected. Inspect for continuity between start stop unit terminal 1Q (wiring harness-side) and BCM terminal 2J (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 1Q and BCM terminal 2J. 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	VERIFY IF MALFUNCTIONING LOCATION IS BCM DEPENDING ON REPEATABILITY 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].) Retrieve the start stop unit DTCs using the M-MDS. (See DTC INSPECTION [START STOP UNIT].) Is the same DTC displayed?	Yes	Replace the BCM, then go to the next step. (See BODY CONTROL MODULE (BCM) REMOVAL/INSTALLATION.)
		No	Go to Step 7.
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].) 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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