P0299 – Turbo/Supercharger Underboost

The ECU expected more boost pressure than it actually measured from the turbo or supercharger system.

Plain‑English Explanation

The ECU commands a target boost level based on load and RPM, then compares this target with the reading from the MAP/boost sensor. If the measured boost is consistently lower than expected, it flags an underboost condition and stores P0299.

Common Symptoms

  • Noticeable loss of power, especially at higher RPM or load.
  • Slower than usual spool‑up from the turbo.
  • Whistling, hissing or whooshing noises from boost pipes.
  • May enter limp mode on some cars.
  • Check engine light on (sometimes only under heavy load).

Typical Causes

  • Boost leaks from intercooler, pipe joints or split hoses.
  • Sticking or leaking wastegate / boost control valve.
  • Faulty boost pressure (MAP) sensor or its wiring.
  • Restricted intake or exhaust, including blocked catalytic converter.
  • Mechanical turbocharger problems (worn bearings, damaged compressor).

What To Check First

  2. Inspect all boost and intercooler pipes for splits, loose clamps or oil staining at joints.
  3. Use smoke or pressure testing equipment to find small leaks.
  4. Check wastegate actuator movement and vacuum/boost lines.
  5. Log commanded vs actual boost with a scan tool on a safe road test.
  6. If mechanical turbo noise or play is present, investigate turbo condition.

P0299 on Ford Fiesta ST & Similar EcoBoost Platforms

On small turbocharged petrol engines like the Fiesta ST 1.6 EcoBoost, underboost is very often down to boost leaks from intercooler pipes and joints, or issues with the wastegate actuator and its control hoses. If you see P0299 alongside lean codes (such as P0171) or misfires (P0300/P0301), prioritise checking for split boost hoses and loose clamps before assuming the turbo itself is worn out.