Diesel Trouble Codes

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

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

Updated September 2, 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 [authoritative facts].
  8. On LMM only, assess DPF restriction as a contributor to underboost; DPF codes apply to LMM only [authoritative facts].

Parts and tools you may need

Rough cost and difficulty

Cost varies by root cause; VGT service, sensor/solenoid replacement, or intake repairs differ. No repair pricing provided.

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.

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) ↩︎

Watch a repair

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

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