P0299 — Turbocharger/Supercharger Underboost
P0299 means the turbo is not delivering the boost asked for. It is usually a leak in the pipework rather than a failed turbo — which is worth knowing before accepting a quote for one.
The engine runs but feels flat, and many vehicles enter a reduced power mode. Driving gently is usually safe, but a turbo failing mechanically can send debris into the engine, so persistent noise alongside this code changes the advice. This code will cause an emissions test failure.
What P0299 means
The module commands a boost level and checks the result against the manifold pressure sensor. P0299 means the measured pressure fell short by more than the allowed margin. Because the charge air travels through several metres of pipework, hoses and an intercooler between the compressor and the engine, a split hose or a loose clamp anywhere along that path produces this code with a perfectly healthy turbocharger. Many vehicles respond by entering reduced power to protect the engine.
| System | Powertrain — forced induction |
|---|
Symptoms you may notice
What causes P0299
Leak in the charge air pipework
A split hose, a perished coupler or a loose clamp lets pressurised air escape before it reaches the engine. This is the most common cause by a wide margin, and often the cheapest possible repair.
Wastegate or variable geometry mechanism stuck
Carbon and heat seize the vane mechanism or the wastegate arm, so the turbo cannot be commanded to build boost. Sometimes freed by cleaning, often needing the actuator or turbo replaced.
Boost control solenoid or vacuum line failed
The solenoid translates the module’s command into a pressure signal at the actuator. A split line or a failed solenoid means the turbo is never asked for boost, however healthy it is.
Turbocharger worn or damaged
Worn bearings, a damaged compressor wheel or a shaft with excessive play mean the turbo cannot make its rated boost. Usually accompanied by noise or oil consumption, and the most expensive of the four.
Common misdiagnosis
Pressure-test the pipework before condemning a turbo. A split coupler produces the same code for a fraction of the cost.
How to diagnose it
Compare commanded and actual boost
Read desired and actual boost in live data under load. The size and shape of the shortfall indicate whether boost is never built or bleeds away as it rises.
Inspect the charge pipework
Follow the path from the compressor through the intercooler to the throttle body. Check every coupler for splits, every clamp for tightness, and look for oil traces that mark where air is escaping.
Pressure-test the charge system
Pressurise the intake tract with regulated air and listen for escapes. This finds leaks that only open under boost, which inspection cannot.
Safety
Use only the low pressure the test specifies. Over-pressurising can burst couplers and the intercooler.
Check the boost control solenoid and actuator
Confirm the solenoid responds to commands and that the actuator moves through its full travel. A seized vane mechanism will resist by hand.
Related codes
Common questions
Do I need a new turbocharger?
Usually not. A split hose or a loose clamp in the charge pipework causes this code far more often than a failed turbo, and costs a fraction as much. Pressure-test before accepting a quote.
Why has the car gone into limp mode?
Many vehicles limit power when boost does not match the command, because the module cannot be sure the engine is safe. It usually clears after a restart, and returns until the fault is fixed.
Is it safe to drive?
Gently, in most cases. But if there is whining, rattling or heavy smoke alongside the code, stop — a turbo failing mechanically can send debris into the engine.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.