Motor Vehicle Expert
OBD fault code guide

P0299 Code Meaning UK

P0299 means “Turbocharger/Supercharger Underboost Condition”. In plain English, the engine computer expected more boost pressure than the turbo actually produced. Common causes include split boost pipes, intercooler leaks, vacuum faults, boost control solenoid issues, sticking turbo actuators, worn turbochargers, MAP sensor faults, EGR issues and DPF restrictions.

📅 Updated June 2026 🇬🇧 UK diagnostic guide 🛠 Turbo underboost fault
Quick answer Fault codes hub Related overboost code
✓ P0299 meaning ✓ Turbo causes ✓ UK repair costs ✓ MOT-aware advice
Quick answer Meaning Symptoms Causes Diagnosis Turbo checklist Repair costs Can you drive? MOT impact Related guides FAQs
Quick answer

What does P0299 mean?

P0299 means the ECU has detected underboost. The turbocharger is not producing enough boost pressure compared with what the ECU requested. Many drivers notice loss of power, limp mode, poor acceleration or the car feeling flat under load. The fault can be caused by something simple like a split boost hose, or something more serious like a worn turbocharger.

Most likely area

Boost pipes, intercooler, vacuum lines, boost solenoid, turbo actuator, MAP sensor, EGR, DPF or turbocharger.

Urgency

Moderate to high. More urgent if there is limp mode, heavy smoke, turbo noise or severe power loss.

MOT risk

Can affect emissions, diesel smoke, engine warning light status and vehicle performance.

Code meaning

P0299 — Turbocharger/Supercharger Underboost Condition

The full description is usually P0299 Turbocharger/Supercharger Underboost Condition. The ECU compares requested boost with actual boost. If actual boost is too low for too long, it stores P0299.

On UK diesel vehicles, this is often linked to boost leaks, vacuum control problems, sticking turbo actuators, EGR faults, DPF restrictions or worn turbochargers. On turbo petrol engines, split boost hoses, diverter valve issues, wastegate problems and MAP sensor faults can also trigger P0299.

Mechanic view

Why P0299 needs proper boost testing

P0299 does not automatically mean the turbo has failed. A small split in an intercooler hose can cause the same code and make the car feel very weak. I would always check pipework, vacuum control and live boost data before pricing a turbo.

The most useful evidence is a road test with live data: requested boost, actual boost, MAF readings, MAP readings and whether the fault happens under load.

Symptoms

Common symptoms of a P0299 code

P0299 usually shows up as poor performance, especially when accelerating, climbing hills or driving at motorway speeds.

Engine management light

A steady engine warning light may appear after the ECU detects low boost pressure.

Limp mode

The car may reduce power to protect the engine and turbo system.

Poor acceleration

The car may feel flat, sluggish or slow to respond when accelerating.

Whistling or hissing noise

A split boost pipe or intercooler leak can sometimes make a hissing or whooshing sound.

Black smoke on diesel

A diesel may smoke if there is not enough air for the fuel being injected.

Fault returns under load

P0299 often returns during hills, overtaking, towing or motorway acceleration.

Common causes

What causes a P0299 fault code?

P0299 is commonly caused by boost leaks, turbo control faults, sensor issues or restrictions in the intake and emissions system.

Very common

Split boost pipe

A cracked, loose or blown-off boost hose can leak pressure before it reaches the engine.

Common

Intercooler leak

A leaking intercooler or damaged end tank can cause low boost and hissing noises.

Vacuum control

Vacuum hose leak

Vacuum-controlled turbos rely on good vacuum pipes and solenoids to move the actuator.

Control fault

Faulty boost control solenoid

A solenoid fault can prevent the actuator or wastegate from controlling boost correctly.

Turbo actuator

Sticking actuator

The actuator may stick, fail electronically or not move far enough to build correct boost.

Turbo wear

Worn turbocharger

A worn turbo may not produce enough boost, especially under load.

Sensor

Faulty MAP sensor

A pressure sensor that reads incorrectly can make the ECU think boost is too low.

Diesel issue

DPF or exhaust restriction

Restrictions can affect turbo performance and cause boost control problems.

EGR-related

EGR fault

EGR flow problems can affect airflow readings and turbo response on modern engines.

Diagnosis

How a garage should diagnose P0299

✅ Scan all stored, pending and permanent codes.

✅ Record freeze frame data before clearing anything.

✅ Check when the fault happens: hills, motorway, towing or hard acceleration.

✅ Compare requested boost with actual boost using live data.

✅ Smoke test or pressure test boost pipework and intercooler.

✅ Inspect vacuum hoses, boost solenoid and actuator movement.

✅ Check MAP and MAF sensor readings.

✅ Check for EGR, DPF or exhaust restriction issues on diesel vehicles.

✅ Assess turbo condition only after external causes are checked.

Avoid this mistake

Do not assume the turbo is dead

A turbocharger is expensive, and P0299 is often caused by something around the turbo rather than the turbo itself. A split intercooler hose, vacuum leak or boost solenoid fault can produce exactly the same low boost symptoms.

Before replacing a turbo, the garage should prove that the turbo is unable to make boost even when the control system, sensors and pipework are working correctly.

Turbocharger diagnostic checklist

Mechanic-style checks for P0299 underboost

A strong P0299 diagnosis checks for lost boost, poor control and false sensor readings. These checks help prevent unnecessary turbo replacement.

Boost leak test

Pressure test or smoke test boost pipes, intercooler and hose connections.

Vacuum test

Check vacuum supply, vacuum hoses, one-way valves and solenoid control.

Actuator movement

Check that the actuator moves smoothly and reaches the correct position.

MAP sensor data

Compare boost pressure readings against expected values and atmospheric pressure.

MAF airflow data

Low airflow readings may support a boost leak, intake issue or EGR problem.

DPF and EGR checks

On diesels, restrictions and EGR faults can affect turbo response and boost control.

UK repair cost guide

How much does P0299 cost to fix in the UK?

Costs vary by vehicle, engine layout, turbo type, diesel or petrol system and whether the fault is a simple boost leak or a turbocharger problem. These are rough UK guide prices only.

Diagnostic scan and road test

Usually around £40–£120 depending on how much live boost testing is included.

Boost hose repair

Can be moderate if a split hose or loose clamp is found early.

Intercooler repair

Cost depends on vehicle access and whether the intercooler or pipework needs replacement.

Vacuum or solenoid repair

Often cheaper than a turbocharger, but diagnosis is needed to confirm the fault.

Turbo actuator repair

Can be more expensive if electronic actuator calibration or replacement is needed.

Turbocharger replacement

Can be expensive due to parts, oil feed checks, labour and access.

Safe driving advice

Can you drive with a P0299 code?

If the car drives normally and the engine light is steady, short careful driving may be possible. Avoid hard acceleration until the fault is diagnosed. If the car is in limp mode, smoking, making turbo noises or badly down on power, get it checked quickly.

✅ Lower risk: stored code only, steady engine light, no smoke and normal power.

⚠️ Medium risk: poor acceleration, hissing noise, occasional limp mode or repeated engine light.

🚫 High risk: heavy smoke, loud turbo noise, severe power loss, repeated limp mode or flashing engine light.

Real experience

What I would check first

On a real car with P0299, I would first check for boost leaks. A split hose underneath the pipe can be hard to see but can cause major power loss. I would also listen for hissing under load and check for oil mist around boost pipe joints.

After that, I would check live boost data, actuator movement and vacuum control. I would only suspect the turbo itself once the pipework, sensors and control system have been tested properly.

MOT implications

Will P0299 fail an MOT?

P0299 can lead to an MOT failure if it causes an emissions-related engine warning light, excessive smoke, poor emissions, limp mode or obvious engine performance problems during the test. On diesel vehicles, low boost can cause poor combustion and smoke.

Smoke risk

Underboost can cause black smoke on diesels because there is not enough air for clean combustion.

Warning light issue

An engine warning light linked to emissions should be fixed before MOT.

Performance issue

A car stuck in limp mode should be repaired before being presented for testing.

Engine light MOT guide Common MOT failures
Back to main hub

Browse more OBD fault code guides

P0299 is one common turbocharger underboost diagnostic trouble code. For more common codes, meanings, symptoms, repair costs and MOT implications, use the main Motor Vehicle Expert fault-code hub.

Back to OBD Fault Codes Hub Use Diagnostic App
Frequently asked questions

Frequently Asked Questions About P0299

Find answers to common questions about P0299 turbo underboost faults, driving safety, repair costs and MOT implications.

What does P0299 mean?

P0299 means the engine computer has detected a turbocharger or supercharger underboost condition. In simple terms, the boost pressure is lower than the ECU expected under load.

Can I drive with a P0299 code?

Short careful driving may be possible if the car drives normally, but you should avoid hard acceleration. If the car is in limp mode, smokes heavily, loses power badly or makes unusual turbo noises, get it checked quickly.

Will P0299 fail an MOT?

P0299 can lead to an MOT failure if it causes an emissions-related engine warning light, excessive smoke, poor emissions, limp mode or obvious engine performance problems.

What causes a P0299 code?

Common causes include a split boost pipe, intercooler leak, vacuum leak, faulty boost control solenoid, sticking turbo actuator, worn turbocharger, MAP sensor fault, EGR issue, DPF restriction or intake leak.

How much does P0299 cost to fix in the UK?

A diagnostic check may cost around £40 to £120. Hose, vacuum or boost leak repairs may be moderate, while turbo actuator or turbocharger repairs can cost significantly more depending on the vehicle.

Is P0299 the same as P0234?

No. P0299 means underboost, where boost pressure is too low. P0234 means overboost, where boost pressure is too high. Both involve turbo boost control but are opposite faults.

About this guide

Based on practical mechanic-style diagnostic experience

Motor Vehicle Expert publishes practical UK vehicle guidance based on real-world mechanical knowledge and hands-on diagnostic experience. This P0299 guide is written to help drivers understand turbo underboost faults before replacing expensive turbocharger parts unnecessarily.

Fault codes should always be treated as a starting point for diagnosis. If your car has heavy smoke, loud turbo noise, repeated limp mode, severe power loss or a flashing engine light, avoid hard driving and get professional help.