P0299 on GMC: How to Diagnose & Fix
Turbo/Supercharger Underboost Condition
3 GMC models · 5 documented issues · 3 critical · $150-$3,500 repair
P0299 on GMC by model: symptoms, diagnosis and fix
The 2018+ Terrain with the 1.5L turbocharged engine (LFV/LYX) experiences PCV (Positive Crankcase Ventilation) diaphragm failure inside the valve cover. The integrated PCV diaphragm tears, causing excessive crankcase pressure that forces oil into the intake manifold and intercooler. Symptoms include oily residue around the air intake and intercooler outlet pipe. In severe cases, excessive crankcase pressure also causes coolant intrusion into the combustion chamber. GM issued TSB PIT5697 for this issue. This is the same problem affecting the Chevrolet Equinox 1.5T of the same era.
Common Symptoms
- Oil residue in intercooler outlet pipe or air intake
- Blue/white smoke from exhaust
- Coolant level dropping with no visible external leak
- Rough idle after cold start
- Check engine light (boost, MAF, or misfire codes)
- Oil smell from engine bay
How to Fix
Replace valve cover assembly with updated GM part (GM 12690058) which includes the revised PCV diaphragm. Clean intercooler and intake manifold of oil contamination. Check for coolant consumption alongside oil intrusion. Inspect air-to-water intercooler for coolant contamination — replacement if contaminated (GM 84446987).
Sources for this issue (1)
On the second-generation Terrain (2018+) with the turbocharged 1.5L LYX or 2.0L LTG engine, the air-to-air charge air cooler (intercooler) accumulates condensate from humid or rainy driving plus PCV-borne moisture. In cold ambient temperatures that water freezes inside the CAC core, restricting charge airflow. GM documents this in service bulletin 18-NA-020, originally issued for 2018-2022 and later expanded through the 2024 model year. Owners on the Terrain/Equinox forum describe ice and standing water inside the charge tube. This is a distinct failure from the CAC outlet duct popping off the throttle body and from the 1.5L PCV coolant/oil intrusion problem - here the cooler itself is obstructed by ice or contamination, not disconnected.
Common Symptoms
- Check engine light in cold weather
- Reduced engine power message
- Rough running and misfire
- Loss of boost / sluggish acceleration
- Water or ice found inside the charge pipe
How to Fix
Follow 18-NA-020: leak-test the CAC, thaw and clean a non-leaking contaminated cooler, and install only the bulletin-specified thermostatic shutter for the confirmed RPO and cold-weather branch. A universal intercooler or grille insert is not the prescribed repair.
Sources for this issue (3)
On 1.5L LYX-equipped Terrains the charge air cooler outlet tube can separate from the throttle body inlet. The retaining clip design does not hold the duct under boost pressure and heat cycling, so the tube pops off, dumping boost and unmetered air. GM issued Service Update N192266090 covering the charge air cooler duct disconnect at the throttle body on 2019 Malibu, Equinox and Terrain 1.5L vehicles, and the condition is further covered by field bulletins PIP5651 and 22-NA-224 across the 2018-2022 range. This is separate from the frozen-CAC condition: the cooler is fine, the connection has come apart. It is also distinct from the throttle body / TPS electrical failure already documented, because the fault is mechanical air-path separation.
Common Symptoms
- Sudden loss of power while driving
- Reduced engine power message
- Check engine light
- Loud whoosh or rushing air sound from engine bay
- Rough idle or stall at low speed
How to Fix
Inspect the outlet duct and throttle-body connection, reseat the duct and install the improved retainer when the duct and sealing surfaces remain serviceable. Replace a damaged duct rather than relying on the clip alone, and use the free service update when the VIN qualifies.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine GM 84828210 charge-air-cooler retainer (supersedes 13434668)$4
2018-2022 GMC Terrain 1.5L turbo branch covered by the service procedure. The OEM catalog also lists the 1.6L diesel, but this issue record is the LYX 1.5L; inspect the duct before ordering.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (3)
The variable geometry turbocharger (Garrett GT3788VA on LML, and similar VGT on L5P) used on 6.6L Duramax engines develops carbon buildup on the variable vanes, causing them to stick and resulting in loss of boost control. Owners report surging, lack of power under load, black smoke, and check engine lights. This is a widely documented issue in Duramax owner communities and referenced in multiple TSBs including 16-NA-026.
Common Symptoms
- Reduced power or hesitation under load, especially above 2000 RPM
- Excessive black smoke from exhaust under acceleration
- Boost pressure gauge reading lower than normal
- Surging or inconsistent throttle response
- Check engine light with boost-related codes
How to Fix
Turbocharger vane cleaning using BG Induction Service or equivalent upper-cylinder cleaner introduced through intake. Severe cases require turbo removal and manual vane cleaning or actuator replacement. Complete turbo replacement with remanufactured Garrett unit ($1,200–$2,500 part) is recommended if vanes are seized. Updated VGT actuator calibration via ECM flash per TSB 16-NA-026.
Sources for this issue (1)
The Garrett variable-geometry turbocharger used on the LBZ, LMM and LML Duramax controls boost by pivoting vanes on a unison ring inside the turbine housing. Soot and carbon bake onto the ring over time and it begins to bind, so the actuator can no longer move the vanes to the commanded position. The ECM sees the mismatch and sets P003A (turbocharger position performance) and/or P2563 (turbocharger vane position sensor performance), usually together, and puts the truck in a reduced-power state. Trucks that idle a lot or run short trips soot the ring up fastest. Duramax forum and Diesel Place threads on these two codes run for hundreds of pages across 2006-2016 Sierra 2500HD trucks.
Common Symptoms
- Check engine light with reduced engine power
- Low boost / sluggish acceleration
- Surging or overboost under throttle
- Poor fuel economy
- Whistle or lack of exhaust brake function
How to Fix
Diagnose which half of the system failed before buying parts. If the vane position sensor alone is faulty it bolts to the turbo actuator and is a comparatively cheap replacement. If the vanes themselves are stuck, the turbo must come off so the unison ring, vanes and turbine housing can be cleaned and freed; badly coked units get replaced. No recall applies. Reducing extended idling and running the engine at load periodically slows re-coking.
Sources for this issue (3)
P0299 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0299 mean on a GMC?
P0299 stands for "Turbo/Supercharger Underboost Condition." P0299 means the engine computer detected an underboost condition — the turbocharger or supercharger is producing less boost pressure than the PCM expects for the given conditions. The computer compares the manifold/boost pressure sensor reading to a target based on engine load, and when actual boost falls short, it sets this code. The most common real-world cause is a leak somewhere in the intake/charge-air path. You'll typically feel reduced power or sluggish acceleration, and the engine may go into a reduced-power mode. On GMC it is documented across 3 models: Terrain, Sierra 3500HD, Sierra 2500HD.
What are the most common causes of P0299?
The most common causes of P0299 are: Boost/intake leak (cracked or loose charge-air hose or intercooler), Faulty or sticking turbo wastegate or actuator, Stuck or clogged variable-geometry turbo vanes (VGT), Faulty boost pressure or MAP sensor, Leaking or disconnected vacuum/boost control lines, Restricted air intake or clogged air filter, Failed boost control solenoid, Worn or failing turbocharger. The specific cause varies by vehicle.
How much does it cost to fix P0299 on a GMC?
Published repair estimates on GMC range from $150 to $3,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0299 on a different make?
View P0299 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.