Wellness a C.E.L. usually only illuminates to indicate an emission fault. The wrench light for a transmission, AWD, throttle body, or possibly a Body Control Module fault.
Underboost doesnt afect any of those so you get a dtc but no CEL. Or that is what I think/understand.
So our cars (at least a 2016) have a waste gate that is normally closed and opened by boost pressure. There is a wastegate control solenoid on the top of the intake to the bottom right of the M.A.P. sensor. This solenoid controls boost pressure to the waste gate actuators to open the waste gates. This in turn controls the ability of the turbos to creat bost.
If that solenoid is disconected and everthing is working the car will over boost because the waste gates are always closed.
The recirculation valves (blow off) prevent compressor stalling when the throttle body closes. (I think) The valves normally prevent boost from recirculating. Power is provided to disturb a bladder and air is recirculated from the cold side cac tubes to the intake side of the turbos.
So, under boost. You could have a problem where a wastegat is not working properly and is sticking open, or one of the c clips came off allowing the wastegate valve in the turbo to do as it pleases.
You could have a mechanical fault with the wastegate solenoid where it is always allowing boost to reach the wastegate actuators.
It is posible that one of your turbos has failed or is failing and unable to produce boost. Our cars can run on one turbo.
Another posible cause is that one of the recirculation valves are not sealing/damage and leaking boost. If you smoke test the intake your not going to find this leak if you plug up the air filter as the valve leaks into the intake from a pressure pipe.
You could have a charge air cooler pipe loose/blown off, but you should hear the air leak.
I think that about covers it?
My car has had an underboost code on it I cleared it goes away ( obviously ) then comes back. This is my first SHO car feels powerful to me I happy as a pig in the S#%T. Took it to the dealer was gonna have diagnostics done to see what the issue was lead guy takes a ride with me an says I would feel a great difference in power with a underboost problem , got no engine light on either. He said it could have been an old code some get stored and stay there. So I take his word and drive on. Now I am happily tuned by AJPTURBO load the tune in and I’m like wow this is great. So I do a little data logging that Brad wanted me to do AAND ! He says I have a big underboost problem not seeing any of the power he is commanding. So I start searching for hoses off things of the norm we should look for. Hoses ripped torn ect... found one thing are these big enough to cause my underboost issues. It’s the charge pipe that runs along top of engine plastic - rubber -plastic and right and the next rubber section I found these. That hose must expand and they gotta open up a lil No ? Lmk
