P0299-What Does An Underboost Code Mean?

Your check engine light is on, your car is not accelerating like it used to, and maybe you hear a hiss, whine, or rattle coming from the engine bay when you’re driving. Unsure of the problem, you get the code read by a mechanic and learn the check engine light is reading out a P0299 code which says your turbo/supercharger has low output. Now what?

What does a P0299 code mean?

A P0299 code is triggered when the ECU of a turbocharged or supercharged vehicle detects an underboost condition under operation. The ECU monitors engine vacuum and intake manifold pressure with a MAP (manifold absolute pressure) sensor and in some cases a boost pressure sensor

What is an underboost condition?

To understand what causes an underboost condition, it is important to know how the charge system works when it is functioning properly. A turbocharger is an air pump driven by exhaust gases, pressurizing the air entering the engine. More air allows more fuel to be burned during a cylinder’s combustion stroke. Superchargers are driven by the engine instead of exhaust gases but also work to pressurize intake air.

Both turbochargers and superchargers allow lower displacement motors to make significantly more power and torque than naturally aspirated engines of the same size and achieve better fuel economy.

An underboost condition occurs when a hardware or software issue prevents the engine from reaching its target boost level. The issue causing the code can be a minor problem taking minutes to fix, or a more severe one requiring an invasive procedure to correct.

What symptoms come with a P0299?

Several symptoms can occur during an underboost condition

  • Loss of power
  • Whining, hissing, or rattling noise from the engine
  • Limp mode
  • Check Engine Light

Loss of Power

The primary symptom seen when a car is throwing a P0299 code is a loss of power. The turbocharger or supercharger is unable to produce the boost pressure requested by the ECU. As a result, the car will feel sluggish and slow compared to when it is functioning normally.

Engine Noise

If the underboost condition is caused by a mechanical failure in the charge system, an audible hissing or whining may be noticed under acceleration. Hissing can indicate pressurized air escaping from the charge system, and a whining noise may indicate a failing or failed turbocharger. A rattling noise may point to a failing or failed wastegate.

Limp Mode

A P0299 code can cause a car to go into limp mode. Limp mode restricts the power output of your vehicle when the ECU senses conditions outside its normal operating range. Limp mode is a safety measure designed to prevent engine damage from occurring while there is an issue present.

Lastly, a check engine light will be on, allowing the P0299 code to be read. 

What is causing the P0299?

Anything preventing the ECU from reading the required amount of boost pressure can trigger a P0299 code. Here are some of the more common causes:

  • Leaking or damaged charge piping
  • Failed diverter valve
  • Failed boost pressure regulation valve
  • Failed turbocharger or supercharger
  • Faulty boost pressure sensor
  • Faulty wastegate/wastegate actuator

Leaking or Damaged Charge Piping

The simplest cause of a P0299 code is a boost leak. Boost leaks occur when pressurized air escapes the charge system AFTER the turbo and BEFORE entering the engine. Boost leaks can happen when charge hoses wear out or at connection points between hoses and other components like the turbo outlet, intercooler, and throttle body. Wear and tear over time can damage these components, and incorrectly connected hoses can come apart, causing a leak. This is a great first place to look when a car throws a P0299 code, as this issue can be resolved relatively quickly and easily.

VW GTI Charge pipes
Charge pipes can become disconnected or wear out, causing a boost leak.

Failed Diverter Valve

Another extremely common cause of underboost conditions, particularly on Volkswagen and Audi vehicles, is a failed diverter valve. The diverter valve is a component in the turbocharger system that recycles pressurized air when the throttle is no longer being applied. When accelerating, the rubber diaphragm in the diverter valve closes, allowing all of the pressurized air to enter the engine. Over time, oil residue and extreme temperatures can cause this diaphragm to tear, allowing pressurized air to leak and preventing the target boost level from being achieved. Replacing the failed diverter valve will allow the engine to make correct boost levels, eliminating the P0299 code.

Diverter valve
A failed diverter valve can prevent the vehicle from achieving its target boost level

Failed Boost Pressure Regulation Valve

The boost pressure regulation valve (commonly called N75 Valve on VW/Audi platforms) controls the opening of the wastegate on the turbocharger, allowing for boost pressure to be safely managed. The wastegate is a valve controlled either mechanically or electrically. It remains closed until a specific boost level is achieved, and then it opens to allow exhaust gasses to bypass the turbine and keep the charge pressure steady. When the boost pressure regulation valve fails, the wastegate defaults to an open position, preventing the vehicle from building any boost pressure. With no other issues present, replacing this valve will restore full power to the car

N75 valve
The N75 valve (VW/Audi platforms) controls when the wastegate opens.

Failed Turbocharger or Supercharger

If the source of pressurized air (the turbocharger or supercharger) fails, the vehicle will be unable to achieve the required boost pressure. Turbochargers spin at very high RPMs and operate under extreme heat conditions. These conditions can cause the turbine shaft seals to wear out and leak oil, and eventually, the turbo will fail. A noticeable whining sound can go along with a failing turbo, and if enough oil is leaking past the seal, black smoke can come out of the tailpipe.

Failed turbocharger
This turbocharger has sustained catastrophic failure

Faulty Boost Pressure Sensor

The boost pressure sensor is typically located on the intake manifold and is responsible for reading the pressure of the intake air after it has passed through the turbocharger. A failed boost pressure sensor will be unable to relay this information back to the ECU and can cause a P0299 and other fault codes to occur.

Failed or Failing Wastegate/Wastegate Actuator

When the wastegate, wastegate actuator, or actuator arm fails to function properly, an underboost condition can occur. Wear and tear can cause play in the wastegate lever, preventing the valve from creating an effective seal. Without a good seal, the turbo will be unable to build boost effectively because some exhaust gas will bypass the turbine.

Play in the lever can prevent the wastegate from closing, causing a P0299 code

The diaphragm in the wastegate actuator can become damaged as well, preventing the wastegate from closing as it should. Damage to the actuator arm can also impact the wastegate’s ability to close, although this is not common.

Wastegate actuator
A torn diaphragm inside the actuator will prevent the wastegate from working

P0299 Codes on Modified Vehicles

P0299 codes occur on stock vehicles frequently. Once a car has been modified and tuned, it will likely be making higher boost pressure than it did when it was stock. A higher boost puts extra stress on all the components in the charge system, increasing the likelihood of failure and an underboost condition. Modifying your car doesn’t guarantee any of these components fail, but the chance of an issue arising will be higher.

Fixing the P0299 Code

With an understanding of the potential causes of an underboost condition, you can begin to examine the components that may have failed. Some causes are relatively easy to fix in the driveway, while others require extensive work and time to complete. If you are unsure of the cause, we can help. We run into cars throwing P0299 codes frequently at Alex’s Autohaus, and we can quickly diagnose the problem and get it repaired.

Schedule an Appointment

Schedule a diagnostic appointment today to figure out what is causing the P0299 code or any other issue with your vehicle. Customers all over the Salt Lake City, Midvale, Sandy, South Salt Lake, and Park City areas trust our technicians to service and repair their European vehicles

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