Turbocharger/Supercharger Boost Control "A" Circuit/Open


The PCM has detected an abnormal reading in the turbocharger or supercharger boost control circuit. The “A” denotes an area of the system circuit, as opposed to a particular malfunction or component.

Code Set Parameters

Sensor input voltage signals (received by the PCM) that indicate a boost level that is less or greater (normally less than 9-pounds or more than 14-pounds) than specified by the manufacturer will cause a code to be stored and a malfunction indicator lamp to be illuminated. The PCM recognizes this data as an inability to effectively control boost levels.


Symptoms may include reduced engine acceleration, whining or rattling noises from the turbocharger or turbo pipes, abnormal smoke from the exhaust, spark plug fouling, excessive engine or transmission temperature, hissing noises from the turbocharger wastegate and/or hoses, and an overall lack of engine performance. Additional codes may include other boost codes, engine misfire codes, or knock sensor codes. Cylinder detonation is another possibility, due to high engine temperatures. The boost pressure gauge (if so equipped) will exhibit abnormal levels of boost pressure.

Common Causes

The most common causes of this code include a vacuum leak at the intake manifold, a dirty air cleaner, the wastegate is either stuck open or closed, a defective intercooler, loose bolts between the exhaust manifold and the turbocharger, a loose flange between the turbocharger and the intake pipe, a faulty boost sensor, shorted or open wiring in the boost sensor circuit, loose, corroded, or disconnected electrical connectors in the boost sensor 5-volt reference circuit, or turbo failure. Turbo failure is usually caused by internal oil leaks and oil supply restrictions. These leaks/obstructions contribute to an insufficient oil supply to the turbocharger main shaft; resulting in a cracked turbo housing, turbo bearing failure, bearing wobble (causing the impeller to contact the housing), and damaged or missing impeller blades.

Common Misdiagnosis

Technicians report that failure to flush all debris and obstructions from the turbocharger oiling system can lead to repeated turbo failure. Turbocharger replacement without exhaustive testing of wastegate function and electrical circuitry is also the cause of frequent mishaps.


  • A proper diagnosis of this code should begin with a basic overview of the turbocharger system
  • Turbocharging is a form of forced air induction
  • 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 are simply engine driven air compressors, designed to accomplish this task
  • 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 turbine 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
  • 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”. As boost pressure rises, engine stress is also elevated
  • Each engine manufacturer provides maximum recommended boost pressure specifications which are programmed into the PCM
  • These specifications are calculated with the purpose of avoiding catastrophic engine failure due to excessive boost pressure or reduced engine performance due to insufficient boost pressure in engines that are equipped with factory forced air induction devices
  • When the limits of these specifications are breached (high or low) a code is stored in the PCM and a service engine soon lamp is illuminated
  • When the code is set and the service engine illuminated, the boost problem should be investigated immediately. Some special tools will be needed to effectively diagnose this code
  • These include (but are not limited to) an OBD-II scanner, a boost pressure gauge, a hand-held vacuum pump, a vacuum gauge, and a dial indicator set. Begin your diagnosis by visually inspecting all wiring and connectors
  • Look for shorted or burned wiring and replace circuitry and connectors as required
  • If the system wiring, connectors, and components appear to be in normal working order, connect the scanner to the diagnostic connector and record all stored trouble 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. Confirm that the engine is in proper working order with no misfires and no engine knocks
  • Next, inspect all turbo hose clamps for tightness and examine turbo intake and intercooler hoses for leaks or cracks
  • Make sure that all air intake hoses are tight and in decent shape
  • If all hoses are tight and in good order and there are no disconnected, torn, or cracked vacuum lines, then firmly grasp the turbo and attempt to “rock” it back and forth on the intake flange
  • If the housing moves at all; tighten the bolts/nuts as required to manufacturer’s torque specifications
  • Place a boost gauge so that it may be observed while actuating the throttle
  • With the engine running in park or neutral, quickly rev the engine to approximately 5,000 RPMs and release the throttle suddenly
  • Observe the boost gauge as boost pressure elevates and see if it exceeds 19-pounds
  • If it does, then you have a wastegate malfunction
  • If boost fails to rise sufficiently (typically 14-pounds), then you have a turbocharger or exhaust problem
  • Wastegate Malfunction: Remove the actuator arm from the wastegate assembly
  • Using the vacuum pump, manually engage the actuator valve and observe the wastegate to make sure that it opens and closes fully
  • Any fluctuation from fully closed will cause a dramatic drop in boost pressure
  • If the wastegate door will not open fully, it could result in low boost pressure. Turbocharger Malfunction: After allowing the engine to cool down, remove the turbo outlet hose and look inside
  • Look for oil standing inside of the housing
  • See if any fins are missing or damaged on the impeller and check for signs that the impeller has been striking or rubbing the inside of the housing
  • Spin the blades by hand and feel for loose or roaring bearings
  • Any of these conditions indicate a faulty turbocharger
  • Install the dial indicator so that it contacts the nose of the turbine outlet shaft and measure endplay
  • Readings that exceed .003 should be considered excessive
  • If the turbocharger and wastegate are functioning properly, find a constant supply of vacuum from the intake manifold and install a vacuum gauge (in-line)
  • With the key on and the engine running (KOER), between 16 and 22-inches of vacuum should be produced by an engine in good working order
  • If vacuum is less than 16-inches a bad catalytic converter may be the culprit. If you still haven’t found an obvious problem, test the electrical circuitry and connectors of the turbocharger boost sensor
  • Confirm voltage and resistance values using manufacturer’s specifications and repair faults as necessary.