Is the speed instability of Audi a6l cold car platform related to voltage?

This is very important. Low voltage will cause various sensor signals to exceed the standard (inaccurate), so the engine CPU cannot accurately control the actuator, so low voltage will cause unstable idle speed. At the same time, low voltage will weaken the ignition energy of spark plug, and will also cause unstable idle speed. As the engine speed increases, the generator voltage increases because the battery is dead.

Generally, when the car is cold, the idle speed is not very stable, and there is obvious jitter, because when the car is cold, the engine emission exceeds the standard and the excess exhaust gas is burned.