Altima Rough Cold Start

That sounds like what happened with my Altima. Cold start problems and rough idling when cold? Have you tried checking the fuel system? You could also perform an injector test. Sometimes the injector gets clogged or starts leaking, messing up the fuel delivery during the cold start phase.

When you say 'injector test', what exactly are you referring to? I've got a multimeter and tools, but I'm no pro. Is there a way to test the injectors without pulling them and sending them to a specialist? And when you say 'fuel system' are you thinking fuel pressure or something else?

Testing the injectors properly can be tricky without the right gear. What I did was go to a garage to have it properly tested and diagnosed. In my case, one of the injectors was faulty, and a new one fixed it.

I was trying to avoid taking it to a workshop, but I think you're right. I'll get the injectors properly tested. It was indeed the injector valve, and it had to be replaced, costing me 515€, but at least it's fixed!