Diesel Trouble Codes

P0299 — Turbocharger/Supercharger Underboost Condition — LLY / LBZ / LMM Duramax

Diagnostic reference for LLY/LBZ/LMM Duramax 2004.5–2010.

Updated October 15, 2026

What it means

Boost pressure is below the minimum expected for current conditions. On LLY/LBZ/LMM Duramax 2004.5-2010, VGT turbo, no SCR/DEF; DPF only on LMM.1

Most likely causes

  1. Turbocharger boost control position sensor/switch circuit range/performance fault — P003A is a valid VGT boost control position sensor/switch circuit range/performance code for this platform. Sensor signal out of range can cause incorrect vane positioning and underboost.231
  2. Turbocharger boost control position sensor circuit high — P2563 is turbocharger boost control position sensor circuit high on LLY/LBZ/LMM. High circuit voltage can cause erroneous position feedback and underboost.31
  3. Boost control solenoid A circuit/open — P0045 is turbocharger boost control solenoid A circuit/open on LLY/LBZ/LMM. Electrical fault in boost control solenoid circuit can prevent proper VGT control and cause underboost.41

How to diagnose

  1. Verify code and freeze frame; confirm P0299 present with VGT platform.
  2. Inspect intake/charge-air plumbing for leaks, loose clamps, damaged boots; check intercooler and piping integrity.
  3. Check turbocharger VGT actuator operation and vane freedom; look for excessive soot/carbon binding vanes.
  4. Test turbocharger boost control position sensor signal and reference/ground; compare to expected range.
  5. Inspect turbocharger boost control solenoid A circuit for opens/shorts and proper command from PCM.
  6. Review related codes: P003A, P2563, P2564, P0045, P0234; these indicate VGT control faults on this platform.
  7. Check MAP sensor readings vs. expected boost under load; verify MAF and MAP inputs used for airflow modeling.
  8. On LMM only, assess DPF restriction as a contributor to underboost; DPF codes apply to LMM only.

Common mistake to avoid

Cleaning or replacing the DPF before checking its pressure sensor and tubes. Cracked, melted, plugged or wet sensor tubes, or a failing sensor, can set DPF codes while the filter itself is fine. They are cheap to check; clear and verify them before touching the filter. Source: FleetRabbit — P2002 DPF efficiency code causes and fixes.

Parts and tools you may need

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
MAP/boost pressure sensor$40–$2000.3–1 hrsEasy DIY
Intercooler boot, clamp or pipe repair$20–$2000.5–2 hrsEasy DIY
VGT actuator / vane control solenoid$100–$4000.5–2 hrsDIY with tools and a manual
DPF professional cleaning (bake and blow)$300–$7001–3 hrsShop

Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If diagnosis indicates internal turbocharger failure, high-pressure fuel system faults, or requires removal of safety-critical components, have a qualified diesel technician perform inspection and repair.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources

  1. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

  3. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎ ↩︎

  4. Curated DTC seed — LLY / LBZ / LMM Duramax (curated) ↩︎

Repair videos: P0299 on the LLY-LMM Duramax

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

How to check for Boost Leaks (Every Duramax Owner Must Watch) — Truck Master · 182K views

How To: Find and Fix Boost Leaks Duramax — Snowblind Productions · 59K views