My 2005 Peugeot 307 won't start in the morning. If I push it out in to the sun for a few hours it will start and then will start for the rest of the day.
I have changed the coolant sensor, changed the ECU and checked the wiring but still can't find the fault.
Do you have any OBD2 codes or warnings on the dashboard? Is this gasoline or diesel?
Assuming gasoline, assuming you hear the engine turning over, no OBD2 codes, generally, an engine requires fuel, compression and ignition to start. It is unlikely compression is going to change by heating it up in the sun for a few hours. It should at least pop if it has fuel, compression and ignition.
That leaves fuel and ignition.
Somewhere on the fuel rail, there is probably a schrader valve, a valve that looks like it is a bicycle tire valve. There should be fuel pressure in this after cranking.
If it is a fuel problem, you should also at least get it to pop with starting fluid.
A coil over plug passive tester can show if the coil is producing a spark. This is the one I use:
https://smile.amazon.com/gp/product/B0021UPE58/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
