fault code guide
P0299: Turbocharger / Supercharger 'A' Underboost Condition
Your engine computer detected lower pressure from the turbocharger than it asked for. You will notice sluggish acceleration, poor pulling power on hills, and possibly limp mode.
How it shows up: A steady amber engine management light usually comes on, often accompanied by a sudden loss of pulling power. You find the specific P0299 fault code by plugging a diagnostic scanner into the car's OBD port.
Updated 9 November 2026
Can I keep driving?
Service soon
The car is drivable for short distances, but driving on low boost strains the engine and risks further damage if the turbo itself is failing mechanically.
What to do right now
- 1Pull over safely if the car suddenly drops into limp mode and will not accelerate properly.
- 2Listen carefully for loud whistling, hissing, or metallic grinding sounds coming from the engine bay.
- 3Check your dashboard to ensure the engine management light is steady and not flashing.
- 4Avoid hard acceleration, heavy towing, or high motorway speeds until the fault is diagnosed.
Likely causes
Most likely first.
- Likely
Split boost pipe or intercooler hose
Flexible rubber or silicone boost hoses crack under heat and pressure over time. Pressurised air escapes before reaching the cylinders, causing the sensor to read underboost.
- Likely
Faulty or stuck turbo wastegate actuator
The actuator controls exhaust gas flow across the turbo turbine. If its electronic motor fails or the vacuum diaphragm splits, exhaust bypasses the turbo and boost cannot build.
- Possible
Cracked or leaking intercooler
Stone chips or corrosion can puncture the intercooler radiator at the front of the car. Escaping charge air prevents the intake manifold from reaching target pressure.
- Possible
Faulty manifold absolute pressure (MAP) sensor
The sensor measures charge air pressure in the intake. If it becomes clogged with oil vapour or its internal circuit degrades, it sends falsely low pressure readings to the computer.
- Possible
Worn or damaged turbocharger unit
Internal turbine bearing wear or oil starvation prevents the turbo compressor wheel from spinning freely at high speed. As a result, the unit cannot physically generate sufficient air pressure.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Listen for air hissing under acceleration
With the car parked and windows down, gently rev the engine while listening near the front. A loud rush of air or blowing hiss points directly to a split charge pipe or loose clamp.
- 2
Inspect the rubber boost hoses
Park safely, turn off the engine, and allow it to cool completely before opening the bonnet. Look and feel along the thick rubber air pipes running between the turbo, intercooler, and intake manifold for visible splits, soft spots, or oily patches.
- 3
Check hose clamps and pipe couplings
Gently wiggle each plastic and rubber air duct connection by hand. Clamps can loosen or blow off completely under pressure, creating a massive boost leak that triggers limp mode immediately.
- 4
Check the turbo actuator linkage
Locate the metal rod connecting the vacuum or electronic actuator to the turbo housing. If accessible, gently test with your fingers to see if the rod moves smoothly or is seized solid.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The total bill depends heavily on whether the leak is simply a split rubber hose or requires a whole reconditioned turbocharger unit.
Intercooler hose or boost pipe replacement
- Do it yourself
- €45–160£40–140 in the UK
- At a garage
- €130–320£110–280 in the UK
Boost control solenoid or MAP sensor replacement
- Do it yourself
- €35–120£30–105 in the UK
- At a garage
- €120–260£100–230 in the UK
Turbocharger wastegate actuator replacement
- Do it yourself
- Garage job
- At a garage
- €280–650£240–570 in the UK
Complete turbocharger recondition or replacement
- Do it yourself
- Garage job
- At a garage
- €850–2,200£750–1,900 in the UK
When to go to a garage
- You hear metallic scraping, grinding, or severe dentist-drill screeching noises from the engine bay.
- Thick blue, grey, or black smoke pours from the exhaust during acceleration.
- The engine management light begins flashing while you are driving.
- The car remains stuck in limp mode and cannot maintain a safe speed on public roads.
Questions drivers ask
Can I keep driving my car with code P0299?
You can drive gently to a local garage or home if the car runs smoothly and there are no metallic noises or heavy smoke. Avoid hard acceleration and steep hills, as an underboosted engine works much harder to maintain speed. If you hear screeching or the engine light flashes, stop driving immediately to avoid catastrophic turbo failure.
Will clearing the P0299 code solve the problem?
Clearing the code will turn off the engine management light and temporarily remove limp mode, but the fault code will return as soon as you accelerate hard. The computer constantly compares actual boost pressure against requested pressure. Until the physical leak, faulty sensor, or stuck actuator is fixed, the underboost condition will trigger again.
How does code P0299 differ from code P0234?
P0299 indicates an underboost condition where the turbo cannot generate the pressure requested by the engine computer. P0234 is the exact opposite, indicating an overboost condition where excessive pressure builds up inside the intake. Overboost is frequently caused by a wastegate stuck closed or seized variable-vane mechanism, whereas underboost usually stems from boost leaks or stuck-open wastegates.
Will code P0299 cause my car to fail the NCT or MOT?
On the UK MOT, a lit engine management light is an automatic failure on petrol cars registered from July 2003 and diesel cars from July 2008. On the Irish NCT, an engine warning light with stored fault codes results in an Advisory Pass under current guidelines, provided tailpipe emissions remain within legal limits. However, low boost often causes incomplete fuel combustion, which can lead to excessive exhaust emissions and cause an emissions test failure on both inspections.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
The warning light you may see
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.