Does P0299 mean my turbo has failed?

Usually not, and it is worth resisting that conclusion until the cheap causes have been checked. P0299 says less boost arrived than was asked for, and the commonest reason by far is that the boost is escaping between the turbo and the engine through a split hose, a cracked intercooler pipe or a clamp that has worked loose. Sticking variable vanes, a faulty boost control solenoid and a blocked particulate filter all come ahead of a worn turbocharger in the order of likelihood.

How do I find a boost leak?

The reliable method is a pressure test, where a garage blanks off the intake, pressurises the charge air system with compressed air and listens or uses soapy water to find the escape. A visual inspection misses a lot, because pipes split on the underside and inside the concertina folds and only open up when flexed or pressurised. If you are checking by eye, flex every rubber section by hand, look for oily dirt marks where air has been blowing out, and check every clamp for tightness.

Will P0299 fail an MOT?

Yes. The engine management lamp is a major defect for diesels first used on or after 1 July 2008 and petrol cars from 1 July 2003, with 371,859 recorded failures against it. Diesels face an additional problem: the smoke opacity test involves hard accelerations to the governed speed, and an engine short of boost burns dirty under exactly that load, so failing the smoke limits as well is a realistic outcome.

Can a blocked DPF cause P0299?

Yes. The turbocharger is driven by exhaust gas, and a heavily loaded particulate filter restricts that flow enough to stop the turbo spinning up properly. It is a circular problem, because low boost makes the engine produce more soot, which loads the filter further. If a diesel has P0299 and a history of short journeys, have the filter's soot loading read before assuming the fault is on the intake side.

Powertrain code · standard

P0299Turbo not producing enough boost

Standard definition: Turbocharger/Supercharger "A" Underboost Condition

Last updated 27 July 2026

Quick answer

The turbocharger is delivering less boost than the engine control unit asked for. Nine times out of ten the turbo is fine and the pressurised air is escaping through a split pipe or a loose clamp before it reaches the engine.

How the car works this out

The control unit commands a boost target and compares it against the pressure the sensor reports, allowing for altitude and temperature. P0299 is set when the measured pressure stays below the target by more than a set margin under load. It is a comparison of what was asked for against what arrived, which is why the diagnosis so often ends somewhere in the pipework rather than at the turbo: a leak between the turbo and the engine means the boost genuinely is being made, it just is not getting there.

What you might notice

Flat, gutless acceleration, especially in higher gears or uphill
Limp mode that clears on a restart and returns under load
A whistling, hissing or chuffing noise under acceleration
Black or grey smoke on a diesel
Higher fuel consumption for the same journey
A car that feels normal around town and hopeless on a motorway slip road

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A split boost hose or intercooler pipe, or a loose clamp letting pressurised air escapeCommon
How to check: A pressure test of the charge air system is definitive and takes a garage minutes. Failing that, inspect every hose along its length and inside the concertina folds, where splits hide until the pipe is flexed.
2Variable turbo vanes sticking with carbon so they cannot close to build boostCommon
How to check: Compare commanded and actual actuator position on live data. This is the same mechanism that causes overboost when it sticks the other way.
3A leaking or cracked intercooler, often from stone damage at the front of the carPossible
How to check: Look for oil staining around the intercooler end tanks and at the bottom of the front bumper.
4A faulty boost control solenoid or a failed actuator diaphragmPossible
5A blocked diesel particulate filter or catalytic converter restricting the exhaust so the turbo cannot spin upPossible
How to check: Check the particulate filter's soot loading on a scan tool, and look at whether the car does mostly short journeys.
6A worn turbo with excessive shaft play or damaged bladesLess common
How to check: Remove the intake pipe and check the compressor wheel for play and for blades that have been struck by debris. This is the expensive answer, and it should be the last one reached, not the first.
7A blocked air filter or crushed intake ductingLess common

What to do about it

1Look for the leak first. A split boost pipe is by a distance the most common cause and among the cheapest repairs on this page, and it is regularly misdiagnosed as a failed turbocharger.
2Have the charge air system pressure tested rather than relying on a visual check. Pipes split on the underside and in the concertina folds where nothing is visible until they are flexed.
3Check the intercooler and its clamps, particularly on a car that has covered motorway miles behind lorries.
4Have the particulate filter loading read on a diesel. A blocked filter stops the turbo working properly and needs a different repair entirely.
5Only investigate the turbo itself once the pipework, the intercooler, the actuator and the exhaust restriction have been ruled out.

Can you keep driving?

Only to a garage

The car is safe enough but noticeably slow, and the limp mode tends to arrive exactly when you need acceleration. There is a second reason not to leave it: on a diesel, an engine running without proper boost burns fuel badly and produces extra soot, which loads the particulate filter faster than it can regenerate. A boost hose left unrepaired can end up costing you a filter.

Will it fail an MOT?

The lamp on its own is a major defect and one of 371,859 recorded failures. For a diesel there is a real second risk: the smoke opacity test requires hard free accelerations, and an engine running without its designed boost burns dirty under exactly those conditions. Underboost is one of the more common reasons a diesel that seems to run acceptably still fails the smoke limits.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Does P0299 mean my turbo has failed?: Usually not, and it is worth resisting that conclusion until the cheap causes have been checked. P0299 says less boost arrived than was asked for, and the commonest reason by far is that the boost is escaping between the turbo and the engine through a split hose, a cracked intercooler pipe or a clamp that has worked loose. Sticking variable vanes, a faulty boost control solenoid and a blocked particulate filter all come ahead of a worn turbocharger in the order of likelihood.
How do I find a boost leak?: The reliable method is a pressure test, where a garage blanks off the intake, pressurises the charge air system with compressed air and listens or uses soapy water to find the escape. A visual inspection misses a lot, because pipes split on the underside and inside the concertina folds and only open up when flexed or pressurised. If you are checking by eye, flex every rubber section by hand, look for oily dirt marks where air has been blowing out, and check every clamp for tightness.
Will P0299 fail an MOT?: Yes. The engine management lamp is a major defect for diesels first used on or after 1 July 2008 and petrol cars from 1 July 2003, with 371,859 recorded failures against it. Diesels face an additional problem: the smoke opacity test involves hard accelerations to the governed speed, and an engine short of boost burns dirty under exactly that load, so failing the smoke limits as well is a realistic outcome.
Can a blocked DPF cause P0299?: Yes. The turbocharger is driven by exhaust gas, and a heavily loaded particulate filter restricts that flow enough to stop the turbo spinning up properly. It is a circular problem, because low boost makes the engine produce more soot, which loads the filter further. If a diesel has P0299 and a history of short journeys, have the filter's soot loading read before assuming the fault is on the intake side.

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