What does it mean?
OBD II fault code P0550 is a generic code that is defined as “Power steering pressure (PSP) sensor/switch circuit malfunction”, and is set when the PCM (Powertrain Control Module) fails to receive a signal from the power steering pressure sensor. Note that this code applies to hydraulic power steering systems; systems that use electric motors to provide steering assistance do not use pressure sensors like the ones found on hydraulic systems.
Modern hydraulic power steering systems are adaptive in the sense that power steering assistance increases at low road speeds, while at high speeds or at speeds above a minimum threshold set by the manufacturer, power steering assistance decreases, or is eliminated completely in order to provide the driver with a positive steering feedback through the suspension and steering systems.
The advantage of this approach lies in the fact that directional control of the vehicle is enhanced, since sudden or exaggerated steering inputs are largely prevented. In practical terms, all power steering systems are fitted with pressure sensors that use a changing resistance to convert the reference signal that enters the sensor into a variable signal voltage, with the degree of change corresponding to a change of pressure in the power steering system.
However, at low engine speeds, the power steering pump represents a significant parasitic drag factor on the engine.
To counter this, the PCM uses the signal voltage from the power steering pressure sensor to calculate an increase in the engine speed, which change is effected by increasing the throttle opening with the throttle actuator on drive by wire systems, or by increasing the idling speed by allowing more air, (and hence more fuel) to enter the engine by commanding the idle air control valve to a larger opening.
In this manner, the PCM prevents the engine from stalling at low speeds, which in turn, allows the engine to overcome the parasitic drag of the power steering pump to be overcome. Additionally, by controlling the engine speed effectively, sufficient steering assistance is generated to make low-speed manoeuvring possible without having to exert undue force on the steering wheel.
In the vent that the PCM does not receive a signal from the power steering pressure switch/sensor, it will set code P0550, and may or may not illuminate a warning light. Also, note that depending on the application, several fault cycles may be required before the code will set or before a warning light is illuminated. The image below shows a simplified schematic of a typical hydraulic power steering system.
Note the location of the pressure sensor in relation to the other major components of the system.