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

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DTC P0850:23 [START STOP UNIT]

Description	Transmission range switch circuit malfunction
Detection condition	The start stop unit detected the following condition 5 times continuously with the ignition switched ON (engine off or on). Transmission range switch is stuck on for a continuous 1 s or more even though the TCM CAN signal is in a position other than the P and N position.
Fail-safe	Perform control of the internal signal for control of the transmission range switch as position other than P, N positions.
Possible cause	DTCs are stored in the TCM. Transmission range switch connector or terminal malfunction PCM connector or terminal malfunction Start stop unit connector or terminal malfunction Short to ground in wiring harness between the following terminals: PCM terminal 2BD—Transmission range switch terminal I Starter relay terminal 1F—Transmission range switch terminal I Transmission range switch malfunction PCM malfunction Start stop unit malfunction

Diagnostic Procedure

Step	Inspection		Action
1	VERIFY TCM DTCs Retrieve the TCM DTCs using the M-MDS. (See ON-BOARD DIAGNOSTIC SYSTEM DTC INSPECTION [TCM (SJ6A-EL)].) Are any DTCs displayed?	Yes	Repair or replace the malfunctioning part according to the applicable DTC troubleshooting. (See ON-BOARD DIAGNOSTIC SYSTEM DTC TABLE [TCM (SJ6A-EL)].)
		No	Go to the next step.
2	INSPECT TRANSMISSION RANGE SWITCH CONNECTOR CONDITION Switch the ignition off. Disconnect the negative battery cable. (See NEGATIVE BATTERY CABLE DISCONNECTION/CONNECTION.) Disconnect the transmission range 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 8.
3	INSPECT PCM CONNECTOR CONDITION Disconnect the PCM 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 8.
4	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 8.
5	INSPECT TRANSMISSION RANGE SWITCH CIRCUIT FOR SHORT TO GROUND Verify that the start stop unit, PCM and transmission range switch connectors are disconnected. Inspect for continuity between start stop unit terminal 1F (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: PCM terminal 2BD—Transmission range switch terminal I Starter relay terminal 1F—Transmission range switch terminal I 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 8.
		No	Go to the next step.
6	INSPECT TRANSMISSION RANGE SWITCH Inspect the transmission range switch. (See TRANSMISSION RANGE (TR) SWITCH INSPECTION [SJ6A-EL].) Is the transmission range switch normal?	Yes	Go to the next step.
		No	Replace the transmission range switch, then go to Step 8. (See TRANSMISSION RANGE (TR) SWITCH REMOVAL/INSTALLATION [SJ6A-EL].)
7	VERIFY IF MALFUNCTIONING LOCATION IS PCM 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].) Switch the ignition ON (engine off). Perform the following work 5 times or more. Operate the selector lever to other than P and N position and wait for 1 s or more. Operate the selector lever to P or N position and wait for 1 s or more. Operate the selector lever to other than P and N position. Retrieve the start stop unit DTCs using the M-MDS. (See DTC INSPECTION [START STOP UNIT].) Is the same DTC displayed?	Yes	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION [SKYACTIV-G 2.0].)
		No	Go to Step 9.
8	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). Perform the following work 5 times or more. Operate the selector lever to other than P and N position and wait for 1 s or more. Operate the selector lever to P or N position and wait for 1 s or more. Operate the selector lever to other than P and N position. 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.
9	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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