Have a 1999 Audi A6 Avant. Has been very hard to start even in good weather. Now, when I go to start it the check engine light comes on with the code reading Manufacturer ignition control. The car runs smoothly overall, no missing or such, just very hard to start. What can cause this? I don't know where to begin with this. Plugs are recent.

 

Sounds like you ignition coil. See link below
Other things that could be causing this issue:
- fuel filter: replace 30 min - search forum for DIY
- vacuum hose leak- search forum for DIY
- air mass sensor: this can be cleaned w/ air mass cleaner see link below
- fuel regulator

CRC 05110 Mass Air Flow Sensor Cleaner - 11 Wt Oz. : Amazon.com : Automotive
2000 Audi A6 Ignition Coil Parts from Buy Auto Parts

Hope this helps - good luck and please post when fixed

 

One other add could be the fuel pump relay switch located under steering wheel just to left