P0299: turbocharger underboost

P0299 means the turbocharger is not producing as much boost as the engine computer asked for. You will usually notice it as lost power, often with the car dropping into limp mode. It is frequently a leak or a sticking actuator rather than a failed turbo. Diagnose soon and drive gently. Indicative UK repair cost is £60 to £1,200.

The most useful thing to understand about this code is that it is not a verdict on the turbo. It is a comparison. The computer asked for a boost figure, the boost sensor reported less than that, and the gap was wide enough and lasted long enough to be logged. Almost anything between the air filter and the inlet valves can open up that gap, and the turbocharger itself is one item on a list it usually does not head.

Can I drive with P0299?

Diagnose soon, drive gently. Limp mode will get you home, but it is not a state to live in, because joining a motorway or completing an overtake with no power available is a safety problem before it is a mechanical one. The mechanical argument for dealing with it quickly is narrower: if the cause turns out to be a turbo with play in its shaft, working it hard risks the compressor wheel touching the housing, and then there is debris heading into the engine.

What P0299 means for your MOT

Boost pressure is not measured at an MOT and there is no defect for a weak turbo. What can fail, in order of likelihood:

  • The lamp. A lamp indicating a malfunction is a major defect under section 8.2.2.2 of the DVSA MOT inspection manual for diesels first used on or after 1 July 2008, and under 8.2.1.2 for petrol cars first used on or after 1 July 2003.
  • The smoke check. A turbo that has started passing oil through worn seals will show as visible blue smoke, and the manual fails a vehicle whose exhaust emits dense blue or clearly visible black smoke. That is a separate defect from the lamp and it is judged by eye.
  • An exhaust leak on the hot side. If the lost boost is coming from a cracked manifold or a blowing joint before the turbo, section 6.1.2 makes an exhaust system with a major leak a defect on its own terms, quite apart from emissions.

There is also a practical trap. The diesel smoke test needs the engine revved freely up to its governed speed, and a car held in limp mode may never get there, which means the test cannot be completed rather than simply failed.

What usually causes it

  1. A boost leak. A split hose, an intercooler pipe that has popped off its clamp, a perished joint, or a cracked plastic charge pipe. Modern engines use a lot of moulded plastic pipework and it splits at the elbows. This is the first thing to look for and the cheapest thing to put right.
  2. Sticking vanes or a sticking wastegate. Variable-geometry turbos carbon up, particularly on cars used mostly for short journeys, until the vanes no longer sweep their full range. The turbo is mechanically sound and simply cannot be commanded. Some free off, some need an actuator, some need the unit.
  3. A faulty boost pressure sensor. The turbo may be making exactly the boost it should and the car cannot see it. Cheap, quick, and worth ruling out before anything is taken apart.
  4. A blocked intercooler or charge pipe. Oil and soot build up inside the intercooler on a high-mileage diesel, and a restricted charge path costs you boost just as effectively as a hole does.

What it costs to fix in the UK

£60 to £1,200. A hose or a charge pipe is the bottom. A boost sensor or an actuator is the lower middle. Cleaning or reconditioning a variable-geometry turbo is the upper middle. A replacement turbocharger is the top, and on engines where it sits behind the engine or under the inlet manifold the labour alone pushes against it.

Given that range, the diagnosis is the part worth paying for properly. See what a car diagnostic costs in the UK for the going rate, and treat any quote for a turbo that arrives without a boost leak test having been done as incomplete.

These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.

What to check first

  1. Listen for it. A boost leak normally announces itself with a whistle or a soft chuffing that rises and falls with the throttle. Under the bonnet, with a helper and the car safely stationary, it is often audible without any equipment at all.
  2. Go round the charge pipework by hand. Every clamp, every elbow, every rubber joint from the turbo outlet through the intercooler to the inlet. Flex the plastic sections rather than just looking at them, because the cracks close up when the pipe is at rest.
  3. Log commanded against actual boost on a full-throttle run. Boost that builds and then falls away sharply is a leak or a wastegate not holding. Boost that never builds at all points at the vanes, the actuator or a restriction.
  4. Check the actuator and the inlet for oil. Confirm the actuator rod moves through its full travel, and look inside the intercooler outlet. Standing oil there tells you about the turbo seals before you spend anything on it.

Related codes

Get it narrowed down before you spend

A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.

Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.