What does it mean?
Depending on the aim, automakers may use forced induction, that is, a turbocharger or supercharger, to dramatically boost the performance of an engine. Forced induction on big V6 and V8 engines can bump engine output to over 1,000 hp. The same technique is used on smaller I3 and I4 engines to boost power for acceleration, while the smaller displacement offers exceptional cruising fuel economy.
Turbochargers and superchargers both function similarly, in that they compress the air going into the intake, though they are driven differently, turbochargers by the exhaust stream and superchargers by a belt or electric motor. More air equals more oxygen, for which the ECM (engine control module) can inject more fuel, therefore extracting more power out of the engine.
Understandably, the ECM needs to modulate how much compression the forced induction system provides, both for performance as well as to protect the engine itself. The wastegate or bypass valve serves this function, bleeding off excess pressure. A turbocharger wastegate refers to the valve that bleeds off exhaust pressure before it has a chance to spool up the compressor.
A bypass valve, which can be used on either turbochargers or superchargers, refers to the valve that bleeds off compressed air in the intake. Either way, intake pressure or boost pressure is limited. Some vehicles use a vacuum- or pressure-operated modulator to control the bypass valve, while others use an electric solenoid valve to control the bypass valve.
On vehicles that use the solenoid valve, the ECM monitors intake pressure, via the MAP (manifold absolute pressure) sensor, modulating the turbocharger bypass valve control solenoid, sometimes called a boost control solenoid (BCS) as needed. It is good to keep in mind that this is a three-piece system. The BCS controls the bypass valve actuator (BVA). The BVA, in turn, controls the boost bypass valve (BBV) inside the turbocharger housing or intake.
If the ECM detects a problem with the BCS circuit, such as an open or short circuit, intermittent connection, or excessive resistance, it cannot control the turbocharger bypass valve. The check engine light (CEL) will illuminate and a diagnostic trouble code (DTC) will be stored in system memory. DTC P0039 is defined as “Turbo/Super Charger Bypass Valve Control Circuit Range/Performance.”