Turbo underboost issue P0299

P.Andy

New Member
Good evening,
i am turning to everyone here that could give me an explanation other then what i have been told so far by 2 mechanics. I have a 2008 Audi A6 C6 2.0 TDI and for the past 2 weeks i keep getting a flashing glow plug light constantly coming up, no EML light though and car also going into a slight state of limp mode. Been already at 2 garages both using a VCDS diagnostic software, on both they got a P0299 fault code "turbo underboost" as a short version. I have changed on the car so far the following; EGR valve, Pressure sensor, Diverter Valve, turbocharger been taken off and investigated (no issues found), changed MAF sensor and finally the fuel filter. If anybody can possibly have any idea of what else could be wrong please help me.

Thank you in advance
 
Good evening,
i am turning to everyone here that could give me an explanation other then what i have been told so far by 2 mechanics. I have a 2008 Audi A6 C6 2.0 TDI and for the past 2 weeks i keep getting a flashing glow plug light constantly coming up, no EML light though and car also going into a slight state of limp mode. Been already at 2 garages both using a VCDS diagnostic software, on both they got a P0299 fault code "turbo underboost" as a short version. I have changed on the car so far the following; EGR valve, Pressure sensor, Diverter Valve, turbocharger been taken off and investigated (no issues found), changed MAF sensor and finally the fuel filter. If anybody can possibly have any idea of what else could be wrong please help me.

Thank you in advance
Check that there is movement in the variable vanes by moving the actuator's arm attached to the turbo. If it's electronic, you can typically remove the c-clip with a flat head to manually move the arm itself without moving the servo and potentially upsetting the software calibration. If it's stiff or difficult to move or not travelling the full range, then that could be causing underboost. If it's vacuum operated, just grab the actuator's arm and give it a push up and down to check for the same.

The next thing would be to check when the last time the car did a regen, which would require diagnostic software plugged in. If it ends up being thousands of miles ago, then it could be that the DPF cycle has stopped, and is consequently getting blocked up more and more, which causes excess pressure, meaning the turbo cannot physically push the air out quick enough, causing an underboost vs the desired amount the ECU wants.
 

P.Andy

New Member
Check that there is movement in the variable vanes by moving the actuator's arm attached to the turbo. If it's electronic, you can typically remove the c-clip with a flat head to manually move the arm itself without moving the servo and potentially upsetting the software calibration. If it's stiff or difficult to move or not travelling the full range, then that could be causing underboost. If it's vacuum operated, just grab the actuator's arm and give it a push up and down to check for the same.

The next thing would be to check when the last time the car did a regen, which would require diagnostic software plugged in. If it ends up being thousands of miles ago, then it could be that the DPF cycle has stopped, and is consequently getting blocked up more and more, which causes excess pressure, meaning the turbo cannot physically push the air out quick enough, causing an underboost vs the desired amount the ECU wants.
Good morning,
Thank you very much for the reply. The actuator has already been checked and it moves freely without any issues also was checked if it holds vacuum and it does. I will have to go back to the garage and see about the regen on the DPF.
 
Top