Start Engine see owners manual
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There is a service bulletin published for this and it is a £0 fix.
This also prevents the Stop/Start from working. "W205 Stop/Start not functioning"
Symptom - Yellow battery symbol fault message in multifunction display "Start Engine. See Owner's Manual" is caused by battery sensor detecting battery resistance of 8 mΩ and above.
Cause - The cause could be an internal resistance problem with the battery but in the case of the common issue reported by the bulletin the cause is: - Excess hot melt adhesive from the heat-shrinkable tube on the cable lug of the measurement line (0.75 mm²) from the on-board electrical system battery G1 (main 12v Battery) to the battery sensor B95 leads to an increase in the internal resistance of the battery. Due to the detected high battery internal resistance (caused by the hot glue), the yellow fault message generated in the multifunction display.
Bulletin, including pictures how to fix are here: MC-10134790-9999.pdf (nhtsa.gov)
Also check ODB2 Codes - Use ODB2 reader to confirm fault code "B1F4500" in the driver side SAM module.
Also check ODB2 real-time data - Use ODB2 reader to view data in driver side front SAM module - check the Stop/Start function data, this lists all the reasons why Stop/Start is inactive - "Battery Condition preventing Stop/Start from operating" will be set to "Active"
Hope this helps






