Opel Corsa 13 Cdti Diesel

Good morning. I have an Opel Corsa diesel CDTI and I can't find the problem. I've used five diagnostic machines, but nothing has worked. The problem is that it's hard to start, as if the injectors were failing, and once it starts, it runs fine. It doesn't run smoothly at high revs, but it's fine in the city. I've also been told it could be the timing belt or the tensioners. What do you think about that?
Thanks.

First, check the fuel pump pressure. If it's okay, use an oscilloscope to verify the compatibility of sensors G28 and G40. It's also possible the engine speed during startup isn't high enough. Does it start well on an incline? It should be approximately above 280 rpm. I don't know if this helps, but that's the method.
Regards.

Check glow plugs, fuel filter (injection pump failure is not common in this type of engine). A common problem is the obstruction of the intake manifold due to exhaust gas recirculation (with soot like chimneys).

Thanks for your reply. I sometimes have to jump-start the car, and I'm wondering if the diagnostic machine will detect the sensor readings or if the data will be stored in the memory at some point. I'll keep investigating. Thanks,

and best regards.

Thanks for your help.
The filter and spark plugs are fine; I'll check the intake to see if I can find anything.
Thanks,

regards