Trouble code

P0299: underboost

The same code points to different failures on different engines. Below, engine by engine, are the documented failures whose records name P0299, with the record's own words, how to tell them apart and the usual fix. The meaning above is taken from the record VGT turbo vane carbon / actuator wear (Garrett VVT).

6.0 Power Stroke (2003-2007)6.7 Power Stroke (2011-present)Duramax LML (2011-2016)Duramax 3.0 (LM2/LZ0)3.5 EcoBoost (gas)

6.0 Power Stroke (2003-2007)

1 documented failure names this code. All about this engine

1 VGT turbocharger sticking

verified How widespread: Common 26 NHTSA owner complaints match it, out of 336 checked

Where its record names the code

overboost or underboost fault codes (P0299, P2262, P2263); occasional limp mode
check for over/underboost DTCs (P0299, P2262, P2263)

Its other signs

low power / sluggish spool-up
surging or hunting boost
loud exhaust 'bark' / sneeze; weak exhaust brake

How to tell it apart

scan tool: compare commanded vs actual VGT duty and boost
check for over/underboost DTCs (P0299, P2262, P2263)
remove and inspect the unison ring for free movement

Usual fix

clean the VGT (soak and manual free-up) as a stopgap

Recalls and bulletins

TSB 08-2-11 TSB 06-... (OASIS 19245)TSB 04-... (OASIS 17472)TSB 03-... (OASIS 16774)

Root cause, parts and sources →

6.7 Power Stroke (2011-present)

2 documented failures name this code. All about this engine

1 Intercooler condensation ingestion + cold-side charge-pipe fatigue

aggregated How widespread: Occasional 6 NHTSA owner complaints match it, out of 1,754 checked

Where its record names the code

underboost code P0299 with a hissing/whooshing sound under load, WITHOUT the turbo bearing-noise signature of VGT turbocharger failure - generation dependent (ceramic-bearing GT32 vs...
pressure-test the charge-air circuit (system capped, soapy water or a smoke machine at the joints) BEFORE condemning the turbocharger for a P0299 - this is one of the most common non-turbo causes of that code on this platform

Its other signs

a momentary shudder, stumble, or brief misfire on hard acceleration after extended light-load driving in humid weather, sometimes with a puff of white vapor from the exhaust
visible oil-misted water at charge-pipe connections on inspection
repeated misfire codes with no corresponding injector-balance fault

How to tell it apart

pressure-test the charge-air circuit (system capped, soapy water or a smoke machine at the joints) BEFORE condemning the turbocharger for a P0299 - this is one of the most common non-turbo causes of that code on this platform
owners frequently mistake the condensation-ingestion stumble for injector or transmission trouble - rule out a corresponding injector-balance fault first
inspect the intercooler end tanks and boots at both ends for oil accumulation, which usually traces back to turbo seal seepage (VGT turbocharger failure - generation dependent (ceramic-bearing GT32 vs...) rather than this mode's own mechanism

Usual fix

replace a failed/split cold-side charge pipe with an upgraded unit

Root cause, parts and sources →

2 VGT turbocharger failure - generation dependent (ceramic-bearing GT32 vs journal-bearing GT37+)

verified How widespread: Occasional 5 NHTSA owner complaints match it, out of 1,754 checked

Where its record names the code

gen2/gen3 (2015-present): underboost code P0299, overboost P0234, actuator fault codes P0045-P0048, sluggish spool without the bearing-failure noise signature
pressure-test the charge-air system BEFORE condemning the turbo for a P0299 - boost leaks (see Intercooler condensation ingestion + cold-side charge-pipe fatigue) mimic turbo failure convincingly on this platform

Its other signs

gen1 (2011-2014): rising high-pitched whine/screech with engine speed, shaft play, oil consumption, blue-gray smoke, eventually wheel-to-housing contact
loss of exhaust-brake-like power under load
oil film or pooled oil in the charge piping (turbo seal transfer, vs normal light crankcase-vapor film)

How to tell it apart

listen at cold start and under load: a healthy turbo whistles evenly; a rising screech or a note that changes pitch under load points at bearings (gen1 signature)
with the intake tract opened, check compressor wheel shaft play - a small amount in/out is normal, side-to-side contactable play is not
pressure-test the charge-air system BEFORE condemning the turbo for a P0299 - boost leaks (see Intercooler condensation ingestion + cold-side charge-pipe fatigue) mimic turbo failure convincingly on this platform

Usual fix

gen1 (2011-2014): replace with an updated steel-bearing turbo assembly, not another ceramic-bearing original-design unit

Root cause, parts and sources →

Duramax LML (2011-2016)

1 documented failure names this code. All about this engine

1 VGT turbo vane carbon / actuator wear (Garrett VVT)

aggregated How widespread: Common

Where its record names the code

underboost codes (P0299), limp mode under load, whistling/whoosh changes

Its other signs

gradual power loss traced to stuck VGT vanes (carbon) or a worn actuator

How to tell it apart

command the vane actuator through its travel with a bidirectional tool
rule out boost leaks (charge-air pipes, clamps) before condemning the turbo

Usual fix

turbo service by diagnosis: actuator/VGT-vane service, charge-air leak repair, or turbo assembly replacement

Root cause, parts and sources →

Duramax 3.0 (LM2/LZ0)

1 documented failure names this code. All about this engine

1 Turbo and boost-path faults (coolant line, charge air)

aggregated How widespread: Occasional

Where its record names the code

underboost/overboost codes (P0299 family), smoke, power loss

Its other signs

coolant loss traced to the hard coolant line to the turbo (community-documented)

How to tell it apart

smoke-test the boost path before condemning the turbo (underboost is often a pipe/clamp)
check the turbo coolant line when coolant loss coincides with turbo-area residue

Usual fix

replace the turbocharger (or have it rebuilt) when the wheel/housing is the fault; the diesel coolant-line rows are not carded in the dealer catalog (the 2.7L gas rows are - do not cross-order)

Root cause, parts and sources →

3.5 EcoBoost (gas)

1 documented failure names this code. All about this engine

1 Turbo / wastegate / boost-control faults

aggregated How widespread: Occasional 13 NHTSA owner complaints match it, out of 667 checked

Where its record names the code

MIL with boost-pressure DTCs (P0236/P0238/P0299/P025E/P007D family)

Its other signs

loss of power / failsafe boost
wastegate rattle at idle or on shutdown
turbo whine or oil traces at the charge pipes

How to tell it apart

scan boost DTCs and desired-vs-actual boost PIDs under load
smoke-test the charge system (pipes, CAC, clamps) before condemning a turbo
listen for wastegate rattle vs phaser/chain noise

Usual fix

charge-system leak repair first (pipes/clamps/CAC), then wastegate actuator / boost sensor; turbo replacement for confirmed bearing/oil-flood cases

Recalls and bulletins

23B65 TSB 22-2198 22U08 15-0071 TSB-13-6-12

Root cause, parts and sources →

A code tells you which system flagged a fault, not which part failed. Read it with the signs your truck shows: symptoms by engine. For your exact truck: check your VIN.

Order: how widespread each failure is in its record (near-universal, very common, common, occasional, rare), then the number of NHTSA owner complaints that match its keywords. Complaint counts are keyword matches, not a manual review of each complaint. A symptom listed here means the failure is documented with that sign; it is not a diagnosis of your truck. Tiers: verified, aggregated, contested, data-gap. How we tier and source facts.