BSI, BSM, ECU relationship?

[Ric Ochet]

I have a cranking but non starting '03 206 s with the 1,4 8v engine. Battery went flat during lockdown, charged it, car didn't start immediately which was odd but after a few tries it fired up fine. Couple of weeks later same story only this time it won't start and I'm sure it used to buzz and click when I connected the battery before. Check plugs, don't seem to be getting sparks or fuel, My cheap OBD reader says it can't connect to the ECU. Tried resetting BSI but only have the spare key with no remote locking so couldn't do it all. Not sure it's that though as the car warns if the key is left in the ignition and the door is opened. Speak to auto electrician, he suggests checking for voltage at the water temperature sensor. Only getting millivolts. So it seems the ECU isn't getting powered. Start reading about BSI and ECU problems but replace BSM with a second hand one from ebay on the off chance it was that. Still no start. Start looking for wiring diagrams and threads on checking the power supplies to BSI and ECU and then came across viewtopic.php?p=613501#p613501 With the ignition on I've got 12v at fuse 4 but nothing on 9 &16 of the black plug or 11 of the grey. Same result with the original BSM. My first thought was I must have been supplied a duff one from ebay but then wondered what the relationship between these units was like. Does for instance the BSM supply power to the BSI that then tells the ECU that there's no security issues, and the ECU then powers up other parts of the BSM or is the BSM completely powered once the ignition is on?
Any help, either practical or correcting a misconception in the above would be greatly appreciated

[wheeler]

The BSM is connected to the BSI via the Body VAN & the ECU is connected to the BSI via the CAN. They all communicate with each other & certain output will only operate when given the right command.
You should have 5v at the coolant temp sensor with the ignition on but you should check it with the plug disconnected.
Has your cheap OBD reader ever connected to the ECU on this car?

[Ric Ochet]

I believe it has although it was so long ago I don't remember the actual occasion. I have tested it on a polo and micra so it's functional. Auto electrician said it should be 5v and yes I did disconnect it to test. Are the pins mentioned on the BSM dependent on a signal from the BSI or ECU or should they be at 12v just from having the ignition switch to on?