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

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DTC B1277:11 [ELECTRICAL SUPPLY UNIT (ESU)]

Description	Back-up light circuit malfunction
Detection condition	Electrical supply unit (ESU) detects a short to ground in the back-up light circuit with the ignition switched ON (engine off or on).
Fail-safe	Not applicable
Possible cause	Back-up light bulb (LH) malfunction Back-up light bulb (RH) malfunction Back-up light (LH) connector or terminal malfunction Back-up light (RH) connector or terminal malfunction Electrical supply unit (ESU) connector or terminal malfunction Short to ground in wiring harness between the following terminals: Electrical supply unit (ESU) terminal 2Q—Back-up light (LH) terminal A Electrical supply unit (ESU) terminal 2Q—Back-up light (RH) terminal A Electrical supply unit (ESU) malfunction

Diagnostic Procedure

Step	Inspection		Action
1	VISUALLY INSPECT BACK-UP LIGHT BULB (LH) Visually inspect the back-up light bulb (LH). Is the back-up light bulb (LH) normal?	Yes	Go to the next step.
		No	Replace the back-up light bulb (LH), then go to Step 7. (See BACK-UP LIGHT BULB REMOVAL/INSTALLATION.)
2	VISUALLY INSPECT BACK-UP LIGHT BULB (RH) Visually inspect the back-up light bulb (RH). Is the back-up light (RH) normal?	Yes	Go to the next step.
		No	Replace the back-up light bulb (RH), then go to Step 7. (See BACK-UP LIGHT BULB REMOVAL/INSTALLATION.)
3	INSPECT BACK-UP LIGHT (LH) CONNECTOR CONDITION Switch the ignition off. Disconnect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Disconnect the back-up light (LH) 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.
4	INSPECT BACK-UP LIGHT (RH) CONNECTOR CONDITION Disconnect the back-up light (RH) 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 ELECTRICAL SUPPLY UNIT (ESU) CONNECTOR CONDITION Disconnect the electrical supply unit (ESU) 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.
6	INSPECT BACK-UP LIGHT CONTROL CIRCUIT FOR SHORT TO GROUND Verify that the back-up light (LH)/(RH) and electrical supply unit (ESU) connectors are disconnected. Inspect for continuity between electrical supply unit (ESU) terminal 2Q (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 the following terminals: Electrical supply unit (ESU) terminal 2Q—Back-up light (LH) terminal A Electrical supply unit (ESU) terminal 2Q—Back-up light (RH) 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. Go to the next step.
		No	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 electrical supply unit (ESU) using the M-MDS. (See CLEARING DTC [ELECTRICAL SUPPLY UNIT (ESU)].) Perform the following operation. Shift the selector lever to the R position. (AT) Shift the shift lever to the reverse position. (MT) Retrieve the electrical supply unit (ESU) DTCs using the M-MDS. (See DTC INSPECTION [ELECTRICAL SUPPLY UNIT (ESU)].) Is the same DTC displayed?	Yes	Repeat the inspection from Step 1. If the malfunction recurs, replace the electrical supply unit (ESU). (See ELECTRICAL SUPPLY UNIT (ESU) 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 [ELECTRICAL SUPPLY UNIT (ESU)].)
		No	DTC troubleshooting completed.

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