Sounds like one of the main 12v leads too/from battery, starter, car earth is loose and /or corroded. This shows up when operating the starter motor as it draws lots of amps and the higher resistance of a faulty connector would drastically reduce the voltage available for other things upstream - eg lights.
Try getting someone to listen with the bonnet up whilst you try to start in fault mode - you may hear a fizzing or other noise from the offending connector. Also follow the main leads from the battery to check if anything is loose. If battery is new make sure the terminal leads are tightened up! Especially check the negative battery to car body earth connection - I've known the crimp connectors to corrode internally. If you try to start a few times the offending connection may become warm.
Edited by Big John on 15/12/2023 at 21:40
|