Looks like the speed sensor:
16705/P0321/000801 - Engine Speed Sensor (G28): Implausible Signal
Engine shuts off
Engine doesn't start
Glow Plug Indicator Lamp (K29) flashes
Engine Speed Sensor (G28) loose
Engine Speed Sensor (G28) faulty
Check Engine Speed Sensor (G28)
The Engine Speed Sensor (G28) is also used as a reference sensor for the crankshaft position and correlates with the Camshaft Position Sensor Bank 1 (G40) and Camshaft Position Sensor Bank 2 (G163).
When an engine will not start due to mechanical timing or spark/fuel related issues it is not uncommon to set G28/G40/G163 fault codes. The basics should be checked if this code returns after extended craning and a no-start with a new sensor installed.
When found in a VW Golf/Jetta (1J/9M) or Audi A3 (8L) with 1.9l TDI-PD (AXR), check TPI 2007067 which explains issues with electro magnetic interference. A modification to the wiring will fix this problem, extensive details can be found in the named TPI. Even though this TPI was originally released for VW Golf/Jetta (1J/9M), a customer of ours came across the same issue on an Audi A3 (8L).
When found in a 2004 VW Golf, Jetta, or New Beetle with 1.9l TDI-PD (BEW), check for TSB 2010461 (01-07-45 formerly 97-05-03) or 201463 (01-07-44 formerly 97-05-04) calling for a flash update to the ECU and a secondary ground installation.