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

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DTC U0401:00/U0401:86 [EVP CONTROL UNIT]

DTC	U0401:00	CAN communication: signal error from PCM
	U0401:86
DETECTION CONDITION		DTC U0401:00 Vacuum sensor 2 signal from the PCM is invalid or not within range for a continuous 1 s. DTC U0401:86 Engine speed signal error from the PCM continues for 5 s.
FAIL-SAFE FUNCTION		Refer to “Fail-safe function table”. (See DTC TABLE [EVP CONTROL UNIT].)
POSSIBLE CAUSE		PCM malfunction
SYSTEM WIRING DIAGRAM		—

Diagnostic Procedure

Step	Inspection		Action
1	VERIFY PCM DTC Using the M-MDS, perform the PCM DTC inspection. (See ON-BOARD DIAGNOSTIC TEST [PCM (SKYACTIV-G 2.0)].) Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE [PCM (SKYACTIV-G 2.0)].) DTC troubleshooting completed, then go to the next step.
		No	Go to the next step.
2	VERIFY DTC TROUBLESHOOTING COMPLETED Using the M-MDS, clear the DTC from the EVP control unit. (See CLEARING DTC [EVP CONTROL UNIT].) Using the M-MDS, perform the EVP control unit DTC inspection. (See DTC INSPECTION [EVP CONTROL UNIT].) Is the same DTC present?	Yes	Repeat the inspection from Step1. If the malfunction recurs, replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION [SKYACTIV-G 2.0].)
		No	Go to the next step.
3	VERIFY NO DTC IS PRESENT Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE [EVP CONTROL UNIT].)
		No	DTC troubleshooting completed.

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