Are there any trouble codes shown (check engine light?). Also, is it a fuel flow/pressure or ignition problem?

Assuming the above are ok, it may be in primary electrical system...If there is a gross defect - the starter may not turn the engine over properly.

With an accurate digital voltmeter you should get the following readings -

at battery terminals - 12.6V with everything switched off (very little or no load on the battery). If less the battery is seriously undercharged, or has an open or shorted cell.

at battery terminals - 13.8 to 14.2V with the car running normally. You should see this at idle with few or no accessories on. It should stay in this range when you rev engine up to 2000 or 2500 rpm, even with a few major accessories (hi beams, heater fan, wipers, etc) turned on. The voltage will decrease within the range at higher temperature (of engine and atmosphere). If lower, the charging system is performing inadequately. If much higher (like 14.5 or 15V), overcharging is occurring.

If the above tests aren't passed - there may be another symptom, like some accessories not working well (including sometimes the stereo), or lights burning out too soon.

You can check at various ciruits on the fuse block (with car running) to see if the voltage is stable and matches closely to what you see at the battery (13.8 to 14.2V).

Though it is sometimes hard to tell with a digital meter - if you see the voltage fluctuating at any point - that would be a possible bad connection or fault pathway.

When testing your electrical system, never disconnect anything while the car is running.