Diesel Trouble Codes

P0299 — Turbocharger/Supercharger Underboost Condition (6.7 Cummins (SCR / DEF))

Diagnostic reference for 6.7 Cummins SCR/DEF 2013–present. Turbo is Holset HE300VG VGT with electric actuator + position sensor; no wastegate/vacuum/boost…

Updated September 2, 2026

What it means

Boost pressure is below the minimum expected for current conditions. ECM detects underboost via MAP and VGT actuator position feedback. On this platform the VGT is HE300VG with electric actuator + integrated position sensor; no wastegate/vacuum/boost solenoid.12

Most likely causes

  1. VGT actuator / position sensor performance or learned travel out of range — Electric VGT actuator sticks, carbon fouling, or position sensor drift causes insufficient vane closure/opening for requested boost. Related codes P226C, P0045, P00AF commonly reported with underboost/limp and exhaust-brake loss.134
  2. Boost leak in charge-air system — Leak or split in intercooler piping, boots, CAC, or intake manifold reduces measured boost. MAP-based control will see low boost vs. commanded VGT position.1
  3. Restricted intake or exhaust airflow — Clogged air filter, intake restriction, or exhaust restriction (DPF/SCR blockage) limits turbo flow and boost. DPF/SCR issues can induce limp/loss of power.156
  4. EGR system restriction or valve stuck open — EGR valve/cooler restriction or valve stuck open dilutes intake charge and reduces boost capability.1
  5. MAP sensor or boost-pressure sensing error — Erroneous MAP reading makes ECM think boost is low. Verify sensor and plumbing before condemning turbo.12
  6. Turbocharger mechanical wear / VGT mechanism sticking — VGT nozzle ring coking/sticking or turbine/compressor wear reduces ability to generate target boost.12

How to diagnose

  1. Verify freeze-frame: load, RPM, commanded vs actual boost, VGT duty/position.
  2. Inspect intake and charge-air system for leaks, loose clamps, damaged boots, intercooler cracks.
  3. Check air filter restriction and intake path for blockage.
  4. Inspect EGR valve operation and cooler for restriction/soot; check for stuck-open condition.
  5. Read VGT actuator actual vs desired position; check for P226C/P0045/P00AF and actuator learned limits.
  6. Monitor MAP sensor voltage and compare to mechanical boost gauge if available; inspect MAP hose for oil/plugging.
  7. Review DPF/SCR status and regen history; excessive regen frequency or DPF restriction can limit boost.
  8. Perform VGT actuator sweep/learn test with scan tool; note position sensor feedback and response.
  9. If no obvious air path faults, evaluate turbocharger mechanical condition and VGT mechanism freedom.

Parts and tools you may need

Rough cost and difficulty

Costs vary by region and labor. VGT actuator replacement is commonly reported as high-cost; turbo replacement is higher. DPF/SCR repairs can be extensive. Use parts/tools list for estimates.

When to see a pro

If underboost persists after air-path checks, or VGT actuator/position faults are present, have a qualified diesel technician evaluate VGT actuator calibration/replacement and turbocharger condition. Do not service high-pressure fuel lines or attempt internal turbo disassembly without proper training. If limp mode occurs while driving, reduce load and seek service promptly.

Sources

  1. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎ ↩︎ ↩︎

  3. Curated DTC seed — 6.7 Cummins (SCR / DEF) (curated) ↩︎

  4. NHTSA owner report 11316016 ↩︎

  5. NHTSA owner report 11477076 ↩︎

  6. NHTSA owner report 11667995 ↩︎

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