Diesel Trouble Codes

P0299 — Turbocharger/Supercharger Underboost Condition (L5P Duramax)

Boost pressure is below the minimum expected for current conditions. On L5P Duramax the turbo is a variable-geometry turbo (VGT) with an electric actuator and…

Updated September 2, 2026

What it means

The ECM detects actual boost is lower than the target for the current operating conditions. On this platform the VGT uses an electric actuator/position sensor and there is no wastegate. Underboost is typically an airflow delivery or VGT control issue, not a fuel-supply issue.12

Most likely causes

  1. VGT actuator/position sensor performance or circuit fault — VGT vane position not matching commanded position; actuator sticking, carbon buildup on unison ring, or position sensor signal out of range/high. Codes that commonly accompany VGT control issues are P003A, P2563/P2564.123
  2. Boost leak in intake tract or charge-air system — Leaks at intercooler pipes, CAC boots, clamps, or cracked plastic intake components reduce measured boost. A boost leak or intake restriction causes underboost (P0299), not overboost.14
  3. Intake restriction / air metering discrepancy — Restricted air filter, clogged intake, or MAF signal out of expected range can limit airflow and boost. MAF circuit range/performance is monitored (P0101).15
  4. Exhaust restriction affecting turbine drive — Restricted DPF/aftertreatment or exhaust blockage reduces turbine energy and boost. Emissions system includes DOC, DPF, and SCR with DEF.15
  5. MAP/pressure sensor or reference issues — Erroneous MAP reading can cause underboost detection even with normal mechanical boost. Air metering uses MAF + MAP for EGR/airflow modeling.15

How to diagnose

  1. Verify freeze frame and repeat under load to confirm underboost vs. sensor anomaly.
  2. Inspect intake and charge-air system for leaks, loose clamps, damaged boots, and intercooler integrity.
  3. Check air filter condition and intake restriction; review MAF readings vs. expected airflow.
  4. Command VGT vane position with scan tool and monitor actual vs. commanded position and actuator duty; inspect for carbon sticking and verify position sensor voltage sweep.
  5. Inspect exhaust backpressure and aftertreatment for restriction; review DPF regen status and differential pressure.
  6. Compare MAP and MAF data to boost target at steady load; check sensor reference voltages and wiring for MAP/MAF.
  7. Review related codes: P003A, P2563/P2564 for VGT control; P0101 for MAF; overboost code P0234 would indicate opposite condition.

Parts and tools you may need

Rough cost and difficulty

Cost varies by root cause. VGT cleaning/replacement, charge-air repairs, sensors, or aftertreatment service can range from modest parts/labor to major component replacement. Diagnosis time is typically 1–2 hours before parts are identified.

When to see a pro

If VGT actuator replacement or turbo removal is required, or if exhaust/aftertreatment restriction is suspected, have a qualified diesel technician perform diagnosis and repair. Do not attempt high-pressure fuel or turbocharger disassembly without proper training and safety controls.

Sources

  1. Curated DTC seed — L5P Duramax (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — L5P Duramax (curated) ↩︎ ↩︎

  3. Curated DTC seed — L5P Duramax (curated) ↩︎

  4. Curated DTC seed — L5P Duramax (curated) ↩︎

  5. Curated DTC seed — L5P Duramax (curated) ↩︎ ↩︎ ↩︎

Watch a repair

Third-party videos, not ours — for illustration. Confirm anything against the factory service information for your truck.

P0299 Trouble Code – Don’t Replace Your TURBO Yet! — 8020 Automotive · 150K views

Causes and Fixes P0299 Code: Turbo / Supercharger Underboost Condition — HVAC Mechanic · 715K views