I have had a C4 (RHR )with the same symptoms.
Yes it was the turbo sensor.

Check live data KOEO
Look for turbo position copy, should read 0%.
Then mityvac turbo wastegate to fully open, this should read 100%.
If more than 10% out ( as Roy mentioned ) the sensor is reading incorrect.

As you can see from the sample of live data below my figure shows turbo position copy at 27%.
I can remember it was showing around 85% at full mityvac wacuum applied.

Dealer can supply wategate actuator complete with sensor, but seeing that the actuator worked ok & held vacuum I just replaced the sensor only - saves a long winded setup precedure to change the whole unit.








0 rpm 736 rpm 2912 rpm Engine speed
0 mg/cp 212 mg/cp 299 mg/cp Air flow measured
21 °C 21 °C 33 °C Inlet manifold air temperature
681 Mbars 1001 Mbars 1363 Mbars Turbo pressure
1235 mbar 1001 mbar 1107 mbar Turbo pressure setting
0 % 87 % 65 % Turbo pressure electrovalve OCR
0 % 87 % 65 % Turbo position setting
27 % 71 % 65 % Turbo position copy
0 % 36 % 50 % EGR valve electrovalve OCR
0 % 36 % 9 % EGR Valve Position Setting
1 % 34 % 1 % EGR valve position repeat
9 % 9 % 9 % EGR throttle electrovalve OCR
5 % 5 % 5 % Inlet air heater throttle OCR




Hope this helps, regards....