P0299Turbocharger underboost

What you will notice

• A clear loss of power, often arriving suddenly on a motorway slip road or a long hill.
• The car dropping into limp mode and refusing to rev past a fixed point until it is restarted.
• Slow acceleration with the engine sounding busy but the car not moving as it should.
• A whistle or a hiss under load if the charge air system is leaking.
• Black smoke on a diesel, because the fuelling was calculated for boost that did not arrive.

Likely causes, most common first

Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.

1. A leak in the charge air pipework or the intercooler

   The most common cause. A split rubber elbow or a clamp that has worked loose lets compressed air escape before it reaches the engine, so the pressure sensor never sees the target.

2. Sticking variable geometry vanes

   Common on diesels that do a lot of short, cold journeys. Soot builds up around the vane mechanism until it no longer moves through its full range.

3. A failed wastegate or actuator control

   Whether the actuator is vacuum operated, pressure operated or electronic, a solenoid or a diaphragm that has failed leaves the wastegate open and the boost never builds.

4. A restricted air filter or a collapsed intake duct

   The turbo can only compress what it can draw. Restriction on the inlet side limits boost as effectively as a leak on the outlet side.

5. A blocked diesel particulate filter or a restricted exhaust

   Backpressure that the turbine cannot work against. Usually accompanied by regeneration warnings and poor economy for some time beforehand.

6. A faulty boost pressure or manifold pressure sensor

   The measurement rather than the boost. Worth ruling out before anything is dismantled, because it is the cheapest outcome.

7. A worn turbocharger

   Last on the list and least common. Shaft play, a damaged compressor wheel or oil starvation damage, usually with noise or oil consumption alongside.

What to check, in order

1. Read commanded boost against actual boost in live data during a full throttle pull.

   Actual well below commanded confirms the code. Actual tracking commanded closely, with the code still setting, moves the suspicion towards the sensor or an intermittent fault under a specific condition.

2. Inspect every joint in the charge air system from the turbo outlet to the intake, including the intercooler.

   Oil film around a clamp usually marks the leak. A charge system that holds pressure clears the whole outlet side.

3. Check the actuator arm moves freely through its travel.

   An arm that moves smoothly when commanded clears the actuator and the vanes. A stiff or stationary arm is the fault, and on a diesel it is usually carbon rather than a failed part.

4. Check the air filter and the inlet ducting.

   Restriction found here explains the code without any turbo work at all.

5. With everything else clear, check the turbo for shaft play and look at the compressor wheel.

   Free rotation with no significant play and an undamaged wheel means the turbo is not the problem, however tempting a conclusion it is.

Where this one turns up most

• Turbo diesels with variable geometry turbochargers used mainly for short journeys, where the vanes never get hot enough to stay clean.
• Any turbocharged vehicle past about eighty thousand miles, where the charge pipe rubber has hardened.

Codes that often show up alongside it

• P0101
• P0171
• P0300