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Loss of turbo boost pressure on 2014 Porsche 911 (991.1) Turbo S OBD-II error code:
Seeking advice on a lack of boost issue with my 2014 Porsche 911 (991.1) Turbo S. I bought it already-modded a few years ago (Titanium exhaust, enlarged intercoolers, Cobb Stage III, cat delete, +100hp).
At VIR a year ago, I lost nearly all turbo boost. Took it to Quantum Speed Works next to the track with OBD-II error codes of P0421 “limit exceeded, primary cat conv bank 1, inadequate effect), P0431 (same, for bank 2) and P0299 (implausible signal, boost pressure control deviation). Quantum looked everything over, and discovered that the rubber coupler to the intercooler Y-Pipe was not securely fastened (the giant worm-drive hose clamp and rubber coupler pipe had slipped off and was leaking). Secured it, and the car ran normally for the rest of the event.
A year and a few hundred miles later, the same problem has returned – loss of turbo boost pressure when driving on the street. Dash gauge shows only ~7 lbs of boost pressure even with the throttle fully open. The CEL is lit (relights immediately after resetting), showing OBD-II error code: P1165 (“Boost Pressure Electric Actuator Driver”). I checked to see if the rubber coupler had loosened off the Y-Pipe again – one side is not flush, but it doesn’t seem like this is the source of the problem. I tried re-securing it as best as my amateur hands could (it took Quantum a good bit of effort to secure it while I watched), and it does seem to be securely covering 80% of the flange. So I’m thinking that it’s not leaking here?
A little google searching on the error code turns this up: “Diagnostic information DME (DFI) control unit; Boost pressure adjuster driver, bank 1 (Electronic adjuster for turbocharger); Possible fault causes: * Loose contact or corrosion on connectors or lines; * Short circuit to B+/ground/between lines or open circuit in the lines; * Boost pressure adjuster (electronic adjuster for turbocharger) faulty. Fault effects: * Boost pressure control switched off (boost pressure low); * Reduced engine torque.”
So, if indeed it’s not a leak at the Y-Pipe and rubber coupler, then maybe . . . a bad actuator? Bad turbocharger? Just looking for some advice or info before I take it to the shop; would like a head-start on possible diagnoses.
Current condition of rubber coupler to Y-Pipe; the gap is as small as I can make it, and the flange is longer than you may think, so hoping that it's not leaking here. This is the BEFORE picture, from last year at Quantum; you can see how far off the rubber coupler is on the flange of the Y-Pipe, before it was pushed back on and the hose clamp tightened.
The poster in this thread, had the P1165 code (among many others). What is interesting is that it was the actuator ... via a bad potentiometer in the actuator.
The actuator in this case is per turbo and changes the vane geometry inside the turbo via a mechanical linkage that the actuator drives and reports position status from. This results in varying boost levels based on the geometry. You might be able to see the connector for each side underneath without removing anything, I don't recall for certain. I do recall that each sides connector faces the front of the car and has a weather boot. if you can see, you might verify that the connector is in place on both sides.
The attached image shows the VTG connector. You can see the male end hanging down by disc.
I don't think you would see a P1165 for a boost loss due to a slipped hose somewhere, in this case some specific condition of the actuator caused that code.
I don't know if there is a PIWIS process to "test" the actuators and report status, but that would be ideal.
I have read (unverified) that the sounds we hear from the rear when we turn on, but not start, are the VTG actuators doing a full range actuation, and then stopping at the vtg target .... have you noticed any difference in those sounds?
I seem to recall that P released an update Y-to throttle body coupler. That is the thing that came off the first time. You might look at getting the revised one as it was changed to minimize slip offs like you had.
Last edited by user 93281574; Nov 6, 2025 at 11:38 AM.
If you floor it and no boost, then its a faulty electronic actuator on either side. The safety measures shuts off both side to protect the turbos.
One simple way to tell is if there is a difference in exhaust sound when you press/activate the 'Sport' button on the centre console. If there isn't, then the electronic actuator is faulty.
Exhaust sounds also more pronounce and noisier (less cleanish) in cabin on comfort mode.
Happened with mine. No boost when foot down. Got P1169 code on Cobb.
Last edited by sirius9; Nov 6, 2025 at 11:41 AM.
Reason: extra info
If you floor it and no boost, then its a faulty electronic actuator on either side. The safety measures shuts off both side to protect the turbos.
One simple way to tell is if there is a difference in exhaust sound when you press/activate the 'Sport' button on the centre console. If there isn't, then the electronic actuator is faulty.
Exhaust sounds also more pronounce and noisier (less cleanish) in cabin on comfort mode.
Happened with mine. No boost when foot down. Got P1169 code on Cobb.
Isn’t most of the sound change (at idle) from normal to sport due to the sound symposer rather than the vtg?
The poster in this thread, had the P1165 code (among many others). What is interesting is that it was the actuator ... via a bad potentiometer in the actuator.
The actuator in this case is per turbo and changes the vane geometry inside the turbo via a mechanical linkage that the actuator drives and reports position status from. This results in varying boost levels based on the geometry. You might be able to see the connector for each side underneath without removing anything, I don't recall for certain. I do recall that each sides connector faces the front of the car and has a weather boot. if you can see, you might verify that the connector is in place on both sides.
The attached image shows the VTG connector. You can see the male end hanging down by disc.
I don't think you would see a P1165 for a boost loss due to a slipped hose somewhere, in this case some specific condition of the actuator caused that code.
I don't know if there is a PIWIS process to "test" the actuators and report status, but that would be ideal.
I have read (unverified) that the sounds we hear from the rear when we turn on, but not start, are the VTG actuators doing a full range actuation, and then stopping at the vtg target .... have you noticed any difference in those sounds?
I seem to recall that P released an update Y-to throttle body coupler. That is the thing that came off the first time. You might look at getting the revised one as it was changed to minimize slip offs like you had.
Wow, super helpful! I took a closer look at the pic of the coupler I posted above, and (miraculously) it's positioned where I can read the part number. It's the 997 part, so presuming that the prior owner updated the coupler when he had the car modded. And given the length of the flange on the IPD plenum in the thread you referenced, I'm reasonably confident that even though the coupler is not fully flush on mine, it's got enough of the flange (and tightened down enough with the clamp) that this isn't the root cause.
So, yeah, methinks that it's an actuator issue, whether with the potentiometer or something mechanical. Will have to check out the sound difference. Thanks very much!
regarding your issue with the throttle body boot, you 100% need to replace that either way. It will continue to fall off more frequently. I had this same problem when i got my first 991.1 turbo S, and its a known issue (especially with IPD y-pipes). You need to replace that boot with the updated, 991.2 design, which will completely stop it from falling off. Im the one that started the thread referenced above by @Rumble-Bee . This is what you need...
regarding your issue with the throttle body boot, you 100% need to replace that either way. It will continue to fall off more frequently. I had this same problem when i got my first 991.1 turbo S, and its a known issue (especially with IPD y-pipes). You need to replace that boot with the updated, 991.2 design, which will completely stop it from falling off. Im the one that started the thread referenced above by @Rumble-Bee . This is what you need...
UPDATE: took it to the shop with your comments & advice, and the shop confirmed that it's a bad Boost pressure actuator ( bank 1 ) left, which needs to be replaced. The price for just the part is $1,695. Is that the going price for one of these? The listed part number is 9A160531503, which shows up as No Longer Available from the usual sources. Anyone have a suggestion for where this part can be found for less? Thanks!
Loss of turbo boost pressure on 2014 Porsche 911 (991.1) Turbo S OBD-II error code:
