P0299 – Turbo/Super Charger Underboost (CDI)
Mercedes CDI P0299 is often caused by control or plumbing issues, especially when the symptom is load-dependent and intermittent.
How it usually shows up
- Flat under load, limp mode on hills or overtakes
- Intermittent power loss with no loud turbo noises
- Historic underboost code after a single event
Most common causes (Mercedes)
- Boost hose leaks/splits (often only open under load)
- Actuator control issues / vacuum leaks (where applicable)
- Sticking VNT mechanism (less common than hoses)
- Airflow restriction or exhaust backpressure affecting boost control
Practical test plan
- Reproduce the fault under load; idle tests can look normal.
- Inspect boost hoses and clamps; look for oil misting at leak points.
- Check actuator movement/control (vacuum or electronic as fitted).
- Verify MAP readings and requested vs actual boost under load.
- If DPF faults exist, consider backpressure effects as secondary causes.
Related reading
CDI DPF & EGR Behaviour
How emissions/regen issues can inhibit performance.
P2002
DPF efficiency – recurrence traps.
Generic P0299 Guide
General underboost tests and common causes.
Tip: If you have multiple codes, start with the one that appeared first, then confirm with live data before replacing parts.