Turbocharger/Supercharger “A” Underboost Condition


A P0299 indicates that a low boost pressure condition has been detected by the PCM.

Code Set Parameters

If the MAP sensor supplies the PCM with a data reading which indicates that turbocharger or supercharger pressure has reached a level that is less than the manufacturer’s predetermined amount, then a trouble code will be stored and a malfunction indicator lamp will be illuminated.


Symptoms include a loss of power, hissing noises from the engine, a stored trouble code, and an illuminated service engine soon lamp. Possible causes include a faulty turbocharger or supercharger,

Common Causes

The most common causes include an obstructed or faulty turbocharger or supercharger, an air restriction or leak, a faulty boost pressure sensor, or a faulty wastegate regulator bypass valve. There are also various motors, actuator arms, diaphragms, and doors that can fail. The boost pressure release valves are often replaced with high-performance adjustable aftermarket valves. This is done to obtain greater turbocharger boost pressure and more power, however the reduced boost readings may cause a P0299 to be stored and a malfunction indicator lamp to be illuminated.

Common Misdiagnosis

The turbocharger and supercharger are often condemned in error when this code is presented. Turbocharger/supercharger failure and wastegate regulator bypass valve failure is the most common cause of underboost.


  • In order to successfully diagnose the forced air induction engine, one must understand a brief overview of the forced air induction system and how under is monitored. Forced air induction is a means of introducing excessive amounts of air into an engine in order to promote gains in horsepower
  • Where a naturally aspirated engine utilizes vacuum created by downward piston movement to draw a controlled fuel/air mixture into the engine’s combustion chambers, the forced air induction engine has air and fuel forced into the combustion chambers using an alternately driven device
  • Turbochargers and superchargers are simply engine driven air compressors, designed to accomplish this task
  • These forced air induction devices are divided into three basic categories: turbochargers, roots type superchargers and centrifugal type superchargers
  • Turbochargers use the pressure from engine exhaust to propel impellers in a two chambered housing
  • The two chambers are totally separate one from another
  • Engine exhaust pressure turns the impeller in chamber “A”, which in turn spins an impeller in chamber “B”
  • The impeller in chamber “B” gathers fresh air through the turbocharger intake system (and intercoolers) and forces the cooler, denser air into the engine
  • Superchargers (both types) are belt driven devices
  • The roots-type supercharger sits on top of the engine and is bolted down in place of the intake manifold
  • The centrifugal type supercharger is mounted on the face of the engine in much the same manner as an air conditioning compressor or an alternator
  • Unlike the turbocharger, which harnesses engine exhaust for propulsion, the supercharger has one chamber
  • Air is drawn into the device, compressed, and forced into engine combustion chambers, using a pair of intertwined independently spinning rotors in the roots type supercharger
  • The centrifugal type supercharger utilizes a centrifugal vane type mechanism to draw air into the housing where it is compressed and reintroduced into the engine as cooler, denser air (in excessive amounts)
  • Superchargers also use intercoolers to decrease air temperature prior to compressing it and forcing it into the engine
  • The cooler that the air temperature can become prior to entering the forced air induction device, the denser it will be when it reaches the combustion chamber
  • Denser air allows fuel to atomize more efficiently and promotes increased horsepower. Obviously, as engine RPM levels rise, forced air induction devices spin faster as well
  • The typical turbocharger doesn’t even begin to “spool up” until the engine reaches 1,700 to 2,500 RPMs and can operate at speeds of 250,000 RPMs under full boost pressure
  • Extreme RPMs are necessary in order for the device to produce air pressure that is greater than that of the atmosphere
  • These elevated air pressure levels are known as “boost pressure”. Each engine manufacturer provides minimum recommended boost pressure specifications which are programmed into the PCM
  • When the limits of these specifications are not met a code P0299 is stored in the PCM and a service engine soon lamp is illuminated
  • A scanner (or code reader) and a digital volt/ohmmeter will be helpful in successfully diagnosing this code-storing condition. Begin your diagnosis with a visual inspection of all wiring and connectors
  • Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
  • Always retest the system after repairs are completed to ensure success. If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. Most underboost malfunctions will be related a faulty boost pressure sensor or a faulty wastegate regulator bypass valve
  • Suspect turbocharger or supercharger failure on high-mileage engines or engines with significant oil consumption problems