P0299 – Turbo Underboost – BMW N54/N55 petrol (BMW N54 3.0 twin turbo)
Fault code P0299 means "Turbo Underboost – BMW N54/N55 petrol". It is common on N54 3.0 twin turbo (135i, 335i, 535i, X6 35i), N55 3.0 turbo (M135i, 335i/435i F-series, 535i, X3/X5 35i) engines. Below are the real-world causes, step-by-step tests and OEM part numbers.
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P0299 quick summary
| Fault code | P0299 |
|---|---|
| Meaning | Turbo Underboost – BMW N54/N55 petrol |
| System | Powertrain (Engine & Transmission) |
| Vehicles | BMW |
| Common on | N54 3.0 twin turbo (135i, 335i, 535i, X6 35i), N55 3.0 turbo (M135i, 335i/435i F-series, 535i, X3/X5 35i) |
| Severity | Medium |
| Can I drive? | Yes, but power is reduced. A large boost leak can make the engine run poorly, so fix it soon. |
| Repair cost | €30 – €2,500 |
Symptoms of P0299
- 'Reduced engine power' / drivetrain warning
- Noticeably less power from mid revs
- Whoosh or hiss under boost
- Rattle from the turbo when lifting off (N54 wastegate)
- Boost lower than requested in live data
Most common causes of P0299 (most likely first)
Mechanic's Pro Tip
On the N54 and N55 the plastic charge pipe after the intercooler often cracks – a cheap check that should come first. Then check the vacuum lines and wastegate control solenoids. On the N54 a rattling turbo on deceleration usually means worn wastegate bushings; that ends in turbo replacement, so confirm all other causes before spending that money.
1.Cracked charge pipe or loose intercooler connection
Most likely2.Leaking blow-off (diverter) valves (N54)
3.Cracked vacuum lines or faulty wastegate control solenoids
4.Worn wastegate bushings / wastegate rattle (N54)
5.Faulty boost pressure sensor or turbo
Step-by-step diagnostic guide
- 1
Read all codes and log requested vs actual boost on a 3rd-gear full-throttle pull.
- 2
Pressure-test the charge air system and inspect the charge pipe and intercooler connections.
- 3
Check vacuum lines, the vacuum reservoir and wastegate solenoids; test the blow-off valves (N54).
- 4
Check wastegate movement and listen for rattle on deceleration.
- 5
Repair the leak or part, clear codes and verify boost on a test drive.
Frequently asked questions
Is it the turbos?
Not usually at first – leaks in the charge pipe and vacuum system are far more common. Turbos are the last item to replace.
Can tuning cause P0299?
Yes, higher boost targets show weak points (charge pipe, blow-off valves) and can trigger underboost codes.
Related fault codes
- P0299Turbocharger Underboost
- P0234Turbocharger/Supercharger Overboost Condition
- P0300Random/Multiple Cylinder Misfire Detected
- 2E81Electric Coolant Pump Fault (BMW Codes 2E81–2E85)
- P0012Intake Camshaft Timing Over-Retarded (Bank 1) – BMW VANOS
- P0016Crankshaft / Camshaft Position Correlation – N47 / N20 Timing Chain
This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.