The N290 fuel metering valve (FMV) for Bosch CP4 high pressure fuel pump (HPFP) equipped TDIs:
All '09+ North American market TDIs run the Bosch CP4 HPFP. While HPFP failure is a documented problem, sometimes these cars set fuel rail pressure faults when failed HPFP isn't to blame. If your common rail TDI has a fault stored for rail pressure yet you don't have evidence of an HPFP failure (glitter under the FMV/N290), the FMV itself is likely the culprit.
Here is the oscillation in rail pressure caused by a failed N290 valve:
Common faults are:
000136 - Fuel Rail/System Pressure - P0088 - 000 - Too High
16471/P0087/000135 == 16471/P0087/000135 - Fuel Rail/System Pressure: Too Low
The Bosch number for this part is 0928400706. Most VAG CP4s have this engraved on top of the valve, but not all.
Installation is a simple 2 bolt affair. The N290 is not available from Volkswagen/Audi. Save nearly $1000 by replacing the N290 yourself instead of paying the dealer to replace the pump.