A6 Audi 01 2.8 won't start, did check the battery.

It absolutely could be the immobilizer (security system). Have you ever had the master key? All the keys should work but if one becomes deprogrammed it needs to be reset (unfortunately by the dealer).
But, before you do that...
Have you checked the voltage level of your battery? It could very well be that your battery has enough juice to power all the accessories but not enough to crank the engine. Try jump starting the car or replacing the battery and see if it starts. Let us know if that works.

when you turn the key do you hear anything from the starter?

try connecting pump direct to battery to see if pump works if it dose then a fuse or relay check all wire connections

it may be the starter motor solenoid that is faulty & needs replacing........cheers.

1995? they did not have the immobilizer installed then, recheck your connections and does the starter turn over? look for open fuses also

The battery is placed on the bottom right side of the plenum chamber for 2001 Audi A6. See picture below:


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Check wire harness between driver door and body for broken wires.Wire harness bends , when door opens and closes so it may brake the wire or two inside harness.

starter soliniod. might need to replace the whole starter

either a dead battery or bad wire connections from battery to starter first check and clean battery connections if still not starting try jumping battery if it still click than you have bad connection at starter

Sounds like the timing is getting out of sync. Check the belt for worn teeth or loose ness.