Originally Posted by queensfield
Yes, the blue wire should be connected. That is how ECU communicates to the IBS. If IBS itself has gone bad which is located directly on the negative terminal, then the car may not start. Often, taking that blue connector out temporarily, may start the car since that stops communication between ECU and IBS. Also, if you ever need to remove IBS to change battery, be very gentle since it is sensitive to mechanical stress and changes its calibration point needed for accurate measurement of battery charge characteristics and its history.
thanks for this, great explanation. Just connected and successfully started car, so no problems so far.