Dealing with a check engine light is never a fun ordeal. That little icon on the dashboard going off is often enough to ruin someone’s day, month, or even a year. There are different tiers of error codes that can trigger a CEL. Some are severe and should be addressed as soon as possible, others are fairly benign. The P0299 check engine light code leans more towards the former than the latter. 

It’s a code that is common for cars that use a forced induction system, and it usually means that something is wrong with that part of your engine. Today, we’ll try to give you all the information you could need in order to troubleshoot, diagnose and potentially fix this code. 

What Is P0299 Code, and What Does it Mean? 

By definition, the P0299 stands for “Turbocharger/Supercharger A Underboost Condition”. In other words, your car’s Engine Control Unit (ECU) has detected that your supercharger or turbocharger isn’t providing enough boost to the engine. The “A” designation is there to differentiate which turbocharger is underboosted in vehicles that have more than one turbo. If your car fits this description, you could potentially see a P02CB error, which often indicates an issue with the other turbocharger. 

Every engine that runs some sort of forced induction, whether it be a turbo or a supercharger, is tuned to be most efficient when these devices are delivering optimal boost. Once you remove a turbo from the equation, your engine’s performance will begin to suffer. Because of this, the symptoms of the P0299 code tend to be rather obvious from the get-go. 

What Are the Common Symptoms of the P0299 Code?

The very first thing you’ll experience when P0299 gets triggered is a check engine light. The ECU will log the code, and you’ll get the infamous orange light on your dash. However, that’s only the first of several symptoms that will appear. Here’s a list: 

• Check engine light
• Significant loss of power 
• Possible limp mode (vehicle dependent)
• Possible rattle from the engine bay that corresponds to turbo/supercharger activation 

Let’s go over each one of these symptoms, minus the self-explanatory CEL, and explain why they happen. 

Significant Loss of Power

When a turbocharged engine loses boost, it will experience a tangible, sometimes significant drop in performance. As mentioned before, these engines are tuned so that they reach peak power and torque only when the forced induction system is doing its job. 

When you remove the boost, you get an underpowered NA engine that is getting a fuel trim that no longer matches its needs. As a result, your car will most likely lose power as soon as P0299 appears in the ECU log. 

Limp Mode 

Limp mode is a fail-safe mechanism used by carmakers to protect the powertrain when the ECU logs a potentially dangerous fault somewhere within the system. 

A loss of boost due to one reason or another definitely falls within the category of a dangerous fault. Some cars will slip into limp mode when they experience too low of a boost in an effort to save the turbo/supercharger and prevent any damage to the engine. 

Rattle From the Engine Bay 

So far we’ve mentioned a few times that the P0299 code could mean that your engine is at risk of serious damage. It’s only fair to explain why. Of all the causes that could lead to this error, the worst one is often accompanied by a rattle coming from within the engine bay. 

If you hear a metallic rattle or similar noise when you see that CEL on your dash, do yourself a favor and pull over as soon as you can. Turbos are extremely delicate devices. Rattle in this context usually means that your turbo or supercharger is falling apart (it’s usually the former). 

The compressor wheel and turbine wheel of a turbo spin at incredible speeds. We’re talking 250,000 RPM and higher for most average turbos. That is insanely fast. When these two components gain any lateral give, they can fall apart in a rather catastrophic way. If you’re lucky, you’ll only have a grenaded turbo to deal with. 

If you’re particularly unlucky, your now disintegrated turbo will send pieces of itself into the engine, thus pretty much causing catastrophic damage to the cylinder walls, pistons, valves, you name it. 

Because of that, if you hear a rattle from the engine bay, pull over until you can figure out what’s going on. 

What Causes the P0299 Error? 

Unlike symptoms, the potential causes of the P0299 code are numerous. So far we’ve only hinted at a catastrophic turbo failure as a cause, but there are several other reasons why you might be losing boost and some of them have nothing to do with the mechanical integrity of your turbo or supercharger. Here’s a list of things that could cause this error: 

• Turbo/Supercharger damage
• Boost leak 
• Bad boost pressure sensor (MAP)
• Bad wastegate/wastegate actuator
• Bad boost controller 

Boost Leak

Boost leak occurs when there is an air leak somewhere in the intake system between where your turbo is and the engine. 

Since just about every setup runs some kind of piping to the engine, it’s not uncommon to have these hoses or pipes crack or to have the clamps/joints come loose where the hose meets the turbo, or where it meets the engine. 

Bad Boost Pressure Sensor (MAP)

Boost pressure sensors or Manifold Absolute Pressure sensor is an essential part of any turbocharged engine. Its job is to measure the amount of boost that is delivered to the engine. The ECU uses this information to adjust anything from fuel trim to wastegate activation. 

Variable geometry turbos use the MAP data to adjust the angle of the fins, which is essential for efficiency in such systems. When this sensor goes, your engine loses some pretty important air intake data. Fortunately, MAP or boost pressure sensors are easy to replace and are often pretty affordable. 

Bad Wastegate/wastegate actuator

When you step on the accelerator pedal in a turbocharged car, the boost pressure builds up in the system. The moment you release the throttle, the throttle body butterfly valve closes, preventing air from entering the engine. 

However, your intake is still pressurized from the turbo buildup. The MAP sensor registers the excess boost pressure, sends that data to the ECU which then tells the wastegate actuator to engage the wastegate flap and relieve some of that pressure from the system. 

A bad wastegate that is stuck in an open position will vent boost even when you’re the throttle is open and the engine needs more air. Your turbo will spin, but no boost will reach the engine, thus causing the underboost condition. 

Bad Boost Controller 

A boost controller is a device that regulates how much boost the turbo is going to make. It does this by telling the wastegate how far it should open. In a sense, it’s a well-regulated air leak that works great as a means of regulating maximum boost values in your system. Boost controller failure can lead to wastegate issues, which can, in turn, lead to the underboost condition. 

How to Diagnose and Fix the P0299 Code? 

The key to diagnosing and fixing the P0299 is to be methodical in your approach. This is one of those diagnostic trouble codes that can have you chasing your tail if you’re not systematic as far as diagnosing the issue goes. Here’s what you need to do: 

• Run a full diagnostic scan
• Inspect the hoses and pipes for leaks
• Inspect the turbocharger
• Inspect the wastegate and the boost controller 
• Check the MAP Sensor

Run a Full Diagnostic Scan 

First thing first, run a full scan of the car. While the P0299 code alone is a good indication of what’s wrong with your car, it may only be a piece of a larger puzzle.

For example, if you get this code, but you also get a P0106 error (Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem), there’s a decent chance that your MAP isn’t working and is the actual cause of your underboost condition. Because of that, run a full scan, assess the situation, and proceed from there. 

Inspect the Hoses/Pipes for Leaks 

Next up, check your intake and turbo hoses. Look for obvious leaks between the manifold and the turbo. Depending on the nature of the leak, it may not be easy to find. In fact, it can be extremely difficult to figure out if your intake is leaking and where. Be patient and use a boost tester if you’re having a particularly hard time with this. 

Inspect the Turbocharger 

If there are no leaks in the system, move on to inspecting the turbo. You’re looking for any signs of turbo failure. These can include the following: 

• Oil leak in/around the turbo
• Turbine/compressor axle wobble
• Cracks in the turbo 

These are the easiest issues to spot. If you own a diesel engine, and you notice oil leaking from the turbo, it’s in your best interest to get it fixed ASAP. Otherwise, you’re risking a “runaway diesel” condition where the oil from the turbo enters the engine and acts as fuel. The only way to turn off the engine in that instance is to suffocate the intake, otherwise the engine ‘runs away’ until the redline RPMs cause it to destroy itself. 

Inspect the Wastegate/Wastegate Actuator 

Inspecting and testing the wastegate and wastegate actuator is easy if you have the right tools. You’ll need a manual air pump with a gauge that can be fitted to the wastegate. You can also use an air compressor if you can dial it down and limit how much pressure it delivers at the gun. 

The idea is to simulate a boost controller by supplying the wastegate with enough pressure to make it open. If there’s no pressure buildup, or if you can actually see inside the turbo where the wastegate actuator is, chances are it’s stuck open. 

Check the MAP Sensor

Last but not least, check the MAP sensor. To do this, you’ll need a multimeter. The idea is to back probe all three wires coming into the MAP connector using your positive probe while grounding the negative. The three wires are the power, ground and the signal wire. 

With the key on and the engine off, back probe the power wire. You should get 5V on the screen. Now, do the same on the ground wire, and your readings should be less than 100 millivolts. If you’re reading more than 100 millivolts on the ground wire, you’re looking at a bad ground somewhere. 

Now that we know that the sensor is powered and is grounded, it’s time to check the signal wire. Back probe that wire the same way and consult your vehicle’s service manual to see what the reference voltage is for the signal wire on the MAP. If the readings differ drastically from what your service manual says, you’re likely looking at a MAP that is toast. 

Can I Drive with a P0299 Code? 

The answer to this question depends on the severity of the issues. However, there are hardly any scenarios where it’s a good idea to be driving a car that’s experiencing an underboost condition. As always, get the car services as soon as possible to avoid more severe repair bills. 

Get Quality Parts for Your Car 

Turbocharger system repairs can be expensive, but they don’t have to be. Here at eEuropart.com, we stock a wide range of spare parts for your car’s forced induction system. To find what you’re looking for, head over to our store section, select your car from the drop-down menu and search for parts that you need. It’s that simple!