Open bonnet and driver's side window (the window is left open for emergency access reasons only whilst battery is disconnected).So very much like a PC should never be shut down by pulling the mains out, the same can be said for a car's controlling computer and network of connected ECUs. Although most of these are robust enough to correct themselves when the power is reapplied, it can sometimes cause 'glitches' in the data network when the ECUs try and reinitialise. Examples of this are the suspension ECU (on a Hydractive Citroen), the fuel injection ECU, Telematics and Head Units, Auto Gear Box, as well as leaving electro valves in a certain position (EGR for example). There is always the possibility that disconnecting a wiring block to an ECU that uses a multiplexed data connection can result in the ECU being left in an 'awakened' state, without having been told to 'sleep' by the controlling ECU like a BSI.ĭepending on the type of ECU, data parameters may not be written to memory if power (as in the battery being disconnected) is suddenly removed. On modern vehicles, the data networks can remain active for some time after the engine is switch off and longer if doors are opened and equipment is being operated.

I've put together a procedure to cover both a BSI reset and for those replacing a battery or needing to disconnect the battery prior to undertaking a repair.įor the purposes of this post I will use the term 'multiplexed' to refer to different data networks and protocols including for example:

Removing battery cover peugeot 307