What does it mean?
OBD II fault code P0235 is a generic code that is (most commonly defined) as “Manifold absolute pressure (MAP) sensor A, TC system circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects a general failure or malfunction in the forced induction boost control circuit that is labelled “A”. Note that “forced induction” can refer to either a turbo charger or a supercharger, both of which are devices that force highly pressurized, additional air into the engine intake system.
NOTE #1: Note that since car manufacturers do not label circuits, sensors, or parts in the same way, it is important to refer to the manual for the affected application to determine exactly which part of the boost control circuit is labelled “A” on that particular application.
Failure to do this could result in confusion and a misdiagnosis, since some applications use multiple pressure sensors in the boost control circuit that all have to supply closely correlating readings for the boost control system to work properly. NOTE #2: While code P0235 is a generic code, it may have definitions that differ in their specific wording from the one given above, depending on the source consulted.
For instance, some sources may list this code as “ Turbocharger/Supercharger Boost Sensor “A” Circuit”, while others may list it as “Turbocharger (TC) boost pressure sensor A/supercharger (SC) boost pressure sensor A – circuit malfunction”. NOTE #3: Note also that on many, if not most VAG group applications, code P0235 may be displayed as 16619/P0235/000565 – “Boost Pressure: Regulation Range not Reached”.
Regardless of the specific wording of the definition though, all definitions mean the same thing, i.e., the PCM is unable to control the boost pressure due to a failure of, or malfunction in, the boost control circuit. Regardless of the specifics of the forced induction device (turbo charger/supercharger) used on any given application, the purpose of the additional, pressurized air is to increase the engine’s power output without using significantly more fuel.
Although forced induction is widely used, the technology is particularly useful on small capacity engines (particularly the small diesel engines in compact cars), since forced induction produces huge power gains at relatively low cost, and without significant penalties in terms of weight and fuel consumption.
However, and again regardless of the specific device involved, the boost pressure from the forced induction device has to be controlled precisely both to derive the maximum benefit from it, and to protect the engine from the extremely harmful (and sometimes fatal) effects of over boost conditions.
While both turbo-, and superchargers use built-in mechanical devices to bleed off excess boost pressure, these devices are electronically controlled by the PCM in such a way that the boost pressure that enters the engine never exceeds the maximum allowable limit. Typically, boost pressure sensors are pressure sensitive, and their resistance changes in direct proportion to changes in pressure.
In practice, these sensors are supplied with a 5-volt reference voltage and a ground that can in some cases, be supplied by the PCM also. In terms of operation, the pressure sensor will allow close to 5 volts to pass back to the PCM via a dedicated signal circuit when no boost pressure present, or when the engine is at idle.
As the boost pressure increases with increasing engine speed however, the sensor’s also resistance increases thereby decreasing the current being passed back to the PCM, which interprets the changing signal voltage as changes in pressure. Note however that on some applications, the boost pressure (and other pressure) sensors are electrically opposite, in the sense that their electrical resistance decreases as the boost pressure increases.
To accomplish effective control of the boost pressure, the PCM uses primary input data from the MAP (Manifold Absolute Pressure) sensor, or a dedicated bo