The car doesn't start well, what should I do?
If there are no other problems (increased fuel consumption, reduced power), then you need to look at the cold start system. I came across two of them:
1) Fuel injection with a separate small nozzle into the air supply system. If the nozzle is old and does not close tightly, gasoline flows out of the supply hose overnight, and when starting the nozzle injects not gasoline, but air. Try to rinse or replace it. Should help.
2) Electronic starting system (Renault Logan). The car is an official one, and I didn't dig into it much, but everything is tied to the idle sensor (officially - IAC, idle speed regulator). We changed it, and it started up well, and the revs stopped "floating". The problem is that you can't get an extra word from the repairmen in the service: all the important ones walk around, as if the spaceships are being repaired, to say the least.
A car can start badly for many reasons - ignition system, power supply system. If the car is modern, then it is best to do computer diagnostics of the car, check, take readings from the sensors. But it is better to do it in one place, and go to repair (fix) in another. So you will avoid unnecessary markups in the repair.
Most often (from experience) - the problem is in the injectors - they are responsible for fuel injection. In second place is the gasoline pump, then the ignition. But on the injector you need a computer to do it right.
So, it is better to go to a computer diagnostics center, but a proven one.