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P0299 – Turbocharger/supercharger underboost

What it means

The engine computer measures boost pressure in the intake with a pressure sensor and compares it with what it is asking the turbo for. If the measured pressure stays below the target for a while, it sets P0299 and often switches to limp mode to protect the engine. The code doesn't say the turbo itself is faulty. It only says the pressure isn't reaching the target.

Symptoms

• Sudden loss of power, especially when accelerating or climbing.
• Check engine light on, possibly limp mode.
• A whistling or hissing noise if boost is leaking.
• On diesels, sometimes smoke.

Common causes (most likely first)

1. A leak on the pressure side: a loose, cracked or split boost hose, or a leaking intercooler.
2. A turbo control fault: a sticking wastegate or variable vanes (VGT), or a bad vacuum hose, solenoid or electric actuator.
3. A faulty or dirty boost pressure sensor, or its wiring.
4. A clogged air filter or a restriction on the intake side.
5. A worn turbo: bearing or blade damage, oil leaks.
6. On diesels, a clogged particulate filter or an EGR fault can play a part.

How urgent is it? Can I drive?

In the short term you can usually reach a workshop in limp mode at moderate speed. Don't push the engine. If you notice blue smoke, oil leaks or a metallic grinding from the turbo, stop. A damaged turbo can push oil into the intake and cause serious engine damage.

What to check first

1. Inspect the boost hoses and clamps from the turbo to the intercooler and the throttle. Oily, sooty patches show where it is leaking.
2. Compare requested and actual boost in live data during an acceleration. If the actual value lags all the way, the fault is real. If the signal jumps around or looks unrealistic, suspect the sensor.
3. Check the vacuum hoses and whether the wastegate/VGT actuator moves, if you can reach it.
4. Check the air filter.
5. See whether EGR, DPF or leak codes are also stored.

When to see a mechanic

If the hoses are intact, pressure-testing the boost system and checking turbo control are workshop jobs. Only replace the turbo once leaks and control faults have been ruled out. A new turbo will still set the code if a leak hasn't been fixed.

FAQ

Does P0299 mean the turbo is bad?

No. The most common causes are a pressure-side leak or a control fault. The turbo itself is just one possibility.

Can a cracked intercooler hose cause it?

Yes, it is one of the most common causes. Escaping pressure stops the turbo from reaching its target.

Can P0299 be a software issue?

On some models the maker releases a software update for boost control. Ask a dealer, quoting your VIN.