Hello,
I have bought second hand 508 SW PHEV 6 weeks ago and when I was inspecting it and trying out while at the dealer the car did not want to enter electric mode, only showing message "electric mode not ready, due to external conditions" (this is translation, my car is in Czech) and the battery was completely drained.
So we charged the battery and the problem stayed. I made some calls and we tried few voodoo tricks but no change.
Then we drove the car to Peugeot service and they actually printed us a Technical service Bulletin (TSB) from 14/04/2023 describing this condition and how to solve it.
I have attached German copy, but in short it says that if the petrol engine has not been heated up for some time, it is forced to do so, to prevent humidity inside the engine and corrosion.
To solve it you should drive 100km steady, engine heats up and unlock pure electric mode.
So I left with the car and after 30 minutes the car turned off petrol engine and used only electric. I used the car 2 days and left for 5 week long vacation with my old car.
So I charged up 508 to 70% before I left.
Here we are, I am back after 5 weeks and the car again refuses to use electric mode only, gasoline engine runs non stop, even when only electric engine is used.
I drove like 6 hours, (3+3 non stop) and it's all the same.
I know only 1 person that have 508 PHEV and he never got into locked into hybrid mode ever and he uses pure electric mode a lot.
Few notes, while we were investigating this issue at the dealership, before we got bulletin, we were told by others various things, such as
- Diagbox did not shown any error or suspicious warning related to the problem
- According to some, if the battery discharges and is kept in such state for long period, Peugeot service must perform some reset via diagnostic to re-enable pure electric (to me this is unlikely, since my car is region-breaking and using electric engine)
- According to some, the message we saw is connected to 12V battery state/health, so we checked main battery and it was OK, but we did not know about the smaller 12V battery in the trunk
- new idea is that the engine has water detection in the oil bucket and the car is now at the oil change interval anyway, maybe that can be preventing turning off the engine
I would be happy and grateful for any information on this, if you encountered it or if you/service fixed it somehow.
licence file was always downloaded from your licence downloader.. device id was taken using 
that also supports lates maps (14.0.0).