Boost control · Fault code

P0299

Turbocharger / Supercharger Underboost

Your ECU asked for boost. The engine did not deliver enough of it, for long enough that the ECU logged a fault.

Severity EXPERT Moderate — diagnose before it gets worse
Driveability EXPERT Drive with caution — the ECU is already protecting itself
Tuning EXPERT Do not tune a car storing this — fix it first A calibration cannot compensate for air that never arrives; tuning onto an underboost fault hides it and moves load elsewhere.

What it looks like in the data

A fault code tells you a threshold was crossed. The shape of the requested-vs-actual boost is what narrows the cause — a sharp spike then a cut is a different fault from a slow shortfall.

Illustrative Requested vs actual boost · not a measurement from any vehicle
0.0 0.4 0.8 1.2 1.6 0s1s2s3s4s5s bar Requested Actual

Example shape: actual builds but plateaus around 0.9 bar and never reaches the 1.40 bar request — the gap is the fault.

Check first — in this order

The order is the product. A list of "possible causes" is what every parts site publishes, and it is why people replace parts blindly. Steps marked by architecture belong to one boost design and are noise on the other.

  1. Pressure-test the charge system cold. It comes first because it is cheap, non-invasive and clears or confirms an entire branch of the tree in one test — not because a leak is the most likely cause. We do not yet have the case data to rank causes, and will not pretend otherwise. EXPERT
  2. Log requested vs actual boost through a full-load pull. The shape of the gap narrows the cause more than any single reading. EXPERT
  3. Variable-geometry (diesel) On a variable-geometry diesel, check the vane mechanism and its actuator for soot binding and sticking before chasing leaks — this is the leading cause on VGT engines and it does not apply to a wastegated petrol engine at all. EXPERT
  4. Wastegated (petrol) Check wastegate / actuator free play and travel, and the electronic actuator adaptation where fitted. EXPERT
  5. Wastegated (petrol) Inspect the diverter or blow-off valve for a torn diaphragm or a stuck piston. EXPERT
  6. Only then assess turbocharger condition — shaft play, wheel damage, oil carry-over. EXPERT

What this code does not license you to replace

  • That the turbocharger has failed. It is the last item on the list, not the first.
  • That the MAP or boost sensor is lying. A sensor fault usually reads implausible, not merely low.
  • That a remap will "get the boost back".

When it shows up with another code

P0299 + P0171 Stored together, these increase the relevance of an intake or charge-air leak, or of another air-measurement and fuelling plausibility problem. They do not identify the cause by themselves — but they do change which test is worth doing first. EXPERT
P0299 + P0101 Both being present shifts weight toward the air path between the sensor and the cylinder head rather than toward boost control. It narrows the search; it does not end it. EXPERT

Companion codes

Codes whose presence alongside P0299 changes the path.

P0234P00AFP0243P0101P0106P2263P0171

Engines we cover that log this

EA888 Gen 3 Wastegated (petrol)BMW B58 Wastegated (petrol)BMW N54 Wastegated (petrol)BMW N55 Wastegated (petrol)BMW N57 Variable-geometry (diesel)VAG EA288 TDI Variable-geometry (diesel)Ford EcoBoost 1.0 Wastegated (petrol)PSA/BMW THP 1.6 Wastegated (petrol)
No workshop sign-off yet. The diagnostic order on this page has had a technical review of the knowledge model, performed by a model — not a manufacturer certification and not a mechanical inspection of any vehicle. No tuner has yet put their name to this ordering. Treat it as a reasoned starting point, confirm with measurement, and get a professional to sign off before you condemn a part.

Built by DMS Tuned

Stuck on P0299? Send us the log.

DMS Tuned is a real ECU-tuning shop. If you have a boost/requested-vs-actual log, run it through the analyser first — then message us and we will read it with you before you spend money on parts.

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