TDI 4.2 EGR/gas recirculation

[vajper]

During the last year I have had the MIL light come on every now and then, caused by EGR errors.

Typically the error codes has been "Insufficient Flow Detected" and "Insufficient Flow - Intermittent" mostly on the right cylinder bank but at some point also on the left one.

I have removed the EGR valves and they looked fine, not much sooth build-up and have also run the output tests on them and notice that they seem to follow the control signal opening and closing.

I also replaced the right EGR valve with a almost-new one that I ran into. Problem is still there and even though I haven't disassembled the return manifolds to ensure they are not clogged, my gut feeling says they are not worse than the valves (which were OK).

I guess there is some sensor that is used to actually measure the return gas flow. I would have guessed a temperature sensor but don't see anything like that in the region around the ERG valve. How is the flow measured?

Any suggestions what to look into?