|Trouble Code||Fault Location||Probable Cause|
|P0550||Power steering pressure (PSP) sensor/switch circuit malfunction||Wiring, PSP sensor/switch, ECM|
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What Does Code P0550 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.
What are the common causes of code P0550 ?
Common causes of code P0550 could include the following-
- Defective power steering pressure sensor/switch
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Low fluid levels caused by leaks in the system
- Mechanical failure of the power steering pump caused by among other factors, low fluid levels, dirty and/or degraded fluid, the use of unsuitable fluid, or the presence of contaminants such as water or mineral oil in the fluid
- Mechanical failure of the rack and pinion assembly or failure of the recirculating ball system in a power assisted steering box. Note that these conditions are often accompanied by trouble in maintaining directional control of the vehicle
- Failed or failing PCM or other controller. Note that these are rare events, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P0550 ?
Common symptoms of code P0550 could include the following-
- Stored trouble code, and possibly an illuminated warning light
- Engine may stall when the steering wheel is turned at low engine speeds
- Depending on the application, the steering wheel may become difficult, or almost impossible to turn at low road speeds or during manoeuvres such as parking, which is normally performed at low speeds
- Depending on the nature of the problem, the power steering pump may emit screeching, whining, whistling, or even tapping sounds when the engine is running, or when the steering wheel is move from side to side
NOTE: In rare cases, there may be no symptoms present apart from a stored trouble code.
How do you troubleshoot code P0550 ?
Record all fault codes present, including pending codes, as well as all available freeze frame data. This information could be useful should an intermittent fault be diagnosed later on.
NOTE: Take note of all additional codes that may be present, and pay particular attention to codes that relate to the engine speed sensor, throttle actuator, and/or the idle air control valve and all control systems that relate to these sensors. Faults and defects in these sensors/control systems can sometimes mimic the symptoms of P0550, which could appear as a result of other codes. If other codes were stored before P0550, repair these codes in the order in which they were stored, since this will sometimes resolve P0550 as well.
Refer to the manual for the application to locate and identify all relevant parts and components. Also, use this opportunity to determine the location, function, color-coding, and routing of all associated wiring and hoses that comprise the power steering system.
As a first step, inspect both the level, and appearance of the fluid in the steering system reservoir. Bear in mind that unless there are fluid leaks in the steering system, the power steering fluid cannot “disappear”, since it is not consumed like engine oil. Therefore, if the level in the reservoir is significantly below the “FULL” mark, there is a leak in the system that MUST be found and repaired before the vehicle is put back into service.
If the fluid is dark in color, or has a “burnt” odor, it is degraded, and must be replaced as per the instructions provided in the manual. Bear in mind that both a low fluid level, and degraded fluid can cause this code, so do NOT skip this inspection.
WARNING: Be aware that most applications require power steering fluid that MUST conform to certain specifications for the system to work reliably and efficiently. NEVER replace power steering fluid with just any “Automatic Transmission Fluid”, because that particular ATF may be formulated in a way that does not provide effective protection against corrosion and/or friction, or worse, may be incompatible with the power steering pump/system design and/or materials used in the pump or system.
In fact, the power steering systems of some applications require highly specialized hydraulic oil, or other types of fluid that is often only available from the authorized dealers. ALWAYS refer to the manual for the application being worked for detailed information on the correct fluid or oil to use when replacing, or topping off the fluid in ANY hydraulic power steering system to avoid the possibility of sudden and possibly catastrophic system failure.
If the fluid level in the reservoir is up to the proper mark, the fluid is in good condition and of the correct type/grade, there are no fluid leaks in the system, and the power steering pump does not emit strange mechanical noises when the engine is running (or when the steering wheel is turned from side to side), perform a thorough visual inspection of all associated wiring and connectors.
Look for damaged, burnt, shorted, disconnected, or corroded wiring and or connectors, and make repairs as required if such damage is found. Clear all code after repairs are complete, and operate the vehicle at very low speed (or turn the steering wheel from side to side with the engine running) to see if the code returns.
If the code persists and all the conditions in Step 3 are met, but there is no visible damage to the control system wiring, prepare to perform reference voltage, continuity, resistance, and ground connectivity checks on all associated wiring. Be sure however to disconnect the sensor from the PCM during this step to prevent damage to the controller. In addition, bear in mind that the pressure sensor/switch forms part of the control circuit, and as such, its internal resistance must be tested as well. Replace the sensor with an OEM part if its resistance deviates from the value stated in the manual.
Compare all other obtained readings with those stated in the manual, and if deviations from stated values are found, repair, or replace wiring to ensure that all electrical values fall within the specified ranges. Clear all code after repairs are complete, and operate the vehicle at very low speed (or turn the steering wheel from side to side with the engine running) to see if the code returns.
In the unlikely event that the code does return after Step 4, resist the temptation to blame either (or both) the throttle actuator, or the idle air control valve, and especially if no codes (active or pending) relating to these are systems present.
Moreover, bear in mind that if the steering wheel is no more difficult to turn at low speed than it was before the code appeared, the problem is unlikely to be related to the pressure developed by the power steering pump. In cases like this, the problem is far more likely to involve intermittent or sporadic electrical issues, which can sometimes be extremely challenging to find and repair definitively. One typical cause of sporadic problems can often be directly traced to cheap, aftermarket pressure sensors of uncertain provenance, and in many cases, the problem can be resolved by replacing the pressure sensor/switch with an OEM part.
Nonetheless, if the fault persists despite multiple repair attempts, obtain a suitable pressure gauge, and perform a test of the actual pressure developed by the power steering pump.
WARNING: Be sure to follow the directions provided in the manual exactly, and observe all safety precautions, since some power steering pumps develop extremely high pressures. Failure to observe basic safety measures (or the provided directions) could cause serious personal injury, as well as significant damage to the power steering system.
Compare the test results with the values stated in the manual, but be aware that replacing a power steering pump on most applications is a technically challenging affair that may defeat most non-professional mechanics. Thus, if the pump is known not to perform to specifications, the better option is to refer the vehicle to the dealer, or another competent repair shop for professional assistance.
Codes Related to P0550
- P0551 – Relates to “Power Steering Pressure Sensor Circuit Range/Performance”
- P0552 – Relates to “Power Steering Pressure Sensor Circuit Low Input”
- P0553 – Relates to “Power Steering Pressure Sensor Circuit High Input”
- P0554 – Relates to “Power Steering Pressure Sensor Circuit Intermittent”