What does it mean?
Like naturally-aspirated engines, forced-induction engines, that is, those equipped with turbochargers or superchargers, need accurate measurements of air flow, temperature, and density, to properly calculate fuel injection and, for gasoline engines, ignition timing.
To that end, the engine control module (ECM) uses various sensors, such as the MAP, MAF, IAT, ECT, CKP, and AFR (manifold absolute pressure, mass air flow, intake air temperature, engine coolant temperature, crankshaft position, and air fuel ratio), to make its calculations on a constant basis. In forced-induction engines in particular, the Vacuum-Boost sensor is critical to properly measuring the air density at any particular moment.
More boost pressure means more air, particularly more oxygen, in the charge. The ECM increases injector pulse, the result being more engine power. In a forced-induction engine, the vacuum-boost sensor is very similar in operation to a MAP sensor, but different in application. Some engines may have both. Both measure pressure, but their signals differ by atmospheric pressure at sea level, 100 kPa or 14.7 psi.
In other words, a MAP sensor with the engine off will read 100 kPa, while a vacuum-boost sensor will read 0 kPa. That way, a forced-induction engine ECM can read both vacuum and pressure, depending on whether or not the turbocharger or supercharger is active. If there is a problem with the boost sensor signal, however, the ECM can’t properly gauge injection and ignition, resulting in poor performance.
Generally speaking, most production vehicles equipped with forced-induction are tuned to somewhere between 9 psi and 14 psi, though each manufacturer has its own specifications. If the ECM detects boost pressure out of the expected range, it will illuminate the MIL (malfunction indicator lamp) and store a diagnostic trouble code (DTC), P0236 “Turbo/Super Charger Boost Sensor ‘A’ Circuit Range/Performance,” in memory.