The MIL warning light came on this morning in my 2003 Audi A6 (67,000 miles) and i took it to advance auto parts and they said the code was P0456 and said that the message it gave them usually meant that there was a small leak in a vacuum hose... does anyone know if this could be right? or what it could be? Any suggestions are appreciated!!! (by the way the gas cap is tight, already checked that).

 

That code doesn't exist in the audi DTC data chart. Make sure you got the correct code or have it scanned again. All the P0400 codes are EVAP related which won't affect engine performance if it was one of those. It strictly emmission related.

 

DTC 16840 (P0456) - EVAP System: Very Small Leak Detected Lower Limit Exceeded – Intermittent

Seems too be mainly EVAP related.

Check your gas cap; and exposed EVAP lines in your engine bay.

Also read through this post...

https://www.audiforums.com/forum/audi-a4-8/cel-p0456-evap-small-leak-troubleshooting-%2Asolved%2A-59466/