What does it mean?
OBD II fault code P0238 is a generic code that is defined as “Manifold absolute pressure (MAP) sensor “A” – TC system high input”, and is set when the PCM (Powertrain Control Module) detects an abnormally high reading or value in the turbocharger’s boost control circuit.
Note that “high input” in this definition could be a higher than expected signal voltage from a defective MAP/boost pressure sensor, or, “high input” could pertain to a higher than expected signal voltage caused by an actual boost pressure that is higher than would be expected given the operating conditions (such as the current engine speed and throttle opening) that obtain at any given moment.
Also, note the following to avoid confusion- The term MAP (Manifold Absolute Pressure) sensor is sometimes used interchangeably with “Boost Control Sensor”, but be aware that some applications use a dedicated turbo -, or supercharger boost control sensor to monitor boost pressure.
In the definition above, “TC”, is an abbreviation for “turbocharger” On many applications, the various sensors in a particular system are labelled “A”, “B”, “C”, and so forth, but be aware that there is no convention that governs how manufacturers label sensors in the same system(s). For instance, one manufacturer may label say, the MAP sensor as sensor “A”, while another may label the same sensor as sensor “B”.
Refer to the manual for the application being worked on to determine how sensors in any given system are labelled.
While the sole function of a turbocharger is to compress the air that enters an engine to increase engine power, the volume and pressure of the compressed air needs to be controlled to within very narrow margins to both ensure that maximum engine power is available over as wide a range of the engine’s power band as possible, and to protect the engine against over boost conditions that can damage, or even destroy the engine.
To accomplish both tasks, engine management systems use pressure sensors in the intake manifold to monitor the total pressure of the intake air. On most applications, this sensor is known as the Manifold Absolute Pressure sensor, and it works by changing its internal resistance in response to changes in the pressure of the intake air.
These sensors are supplied with a 5-volt reference voltage by the PCM, and as the pressure in the manifold rises, the resistance in the sensor increases by a predictable amount, which causes less current to flow back to the PCM, and vice versa , as the pressure decreases. NOTE: On some applications, MAP, or boost pressure sensors are electrically opposite; refer to the manual for the application for detailed information on this very important point.
This return current is known as the signal voltage, which the PCM interprets as pressure. The PCM uses this information to control a dumping mechanism, known as a “waste gate” in the turbocharger that controls, or limits the volume of exhaust gas that is used to drive the turbochargers’ turbine wheel.
Thus, if the MAP sensor signals the PCM that the maximum allowable boost pressure had been reached, the PCM delivers a signal to the waste gate actuator to open the mechanism to allow some of the exhaust gas, also referred to as “drive pressure”, to bypass the turbine wheel by flowing directly into the exhaust system. Conversely, in order to maintain the boost pressure at optimum levels, the PCM commands the waste gate closed when the pressure in the manifold drops.
In a fully functional system, the PCM keeps the boost pressure within the permissible range by continually controlling the movement of the waste gate, with effective control relying on a continuous signal from the MAP sensor, which signal will continuously increase and decrease in direct response to the continually changing engine speed that is a feature of normal driving.
However, due to harsh operating conditions, the waste gate mechanism is prone to failures such as binding, sticking, or even fracturing. When this h