As Lee has said earths wiring and fuses.
I had a Renault Espace recently from another mechanic who was struggling to fix a non starter.(was all but ready to either buy a new key + coding or even scrap the vehicle "year 2000")
The owner said he had stalled it at a junction and when trying to re-start it just turned over but wouldn't start, When plugged into diagnostic equipment every thing was pointing to an immobiliser fault but turned out to be a blown 70 amp Maxi fuse for either Aircon or dual charger as fuse description (can't remember which). When replaced all was good again.
You can definitely watch time tick by on these jobs.
Ian