Powertrain, generic · Vehicle speed, idle control and auxiliary inputs

P0551 Power Steering Pressure Sensor/Switch Circuit Range/Performance

Check engine lightUsually off
EmissionsRelevant
Drive cycleNot required
Limp modePossible

Hydraulic power steering has a pressure switch or sensor on the high-pressure line; when the wheel is turned at standstill, pump pressure and engine load rise sharply, so the engine module raises idle air in advance to stop the engine stalling. That makes it an input to idle control under CARB. Switches (Saab, Mitsubishi) ground a pull-up; analogue 5 V sensors (Toyota) give about 0.45 V at zero pressure. GM's and Ford's tables do not use P0550–P0554. P0551 means the switch reports high pressure where that is implausible – at speed there is hardly any steering. Mitsubishi sets it when the switch stays on through 10 drive-and-stop cycles; Saab's Trionic T8 after 600 s above 50 km/h, keeping raised idle as substitute and without warning lamp.

Likely causes

Pressure switch stuck closed or faulty internally
Signal line shorted to ground (ground means active)
Permanently high pressure (steering valve sticking, kinked line)
Vehicle speed signal faulty (Saab depends on P0501)

Symptoms

  • Permanently slightly raised idle
  • No load compensation when steering
  • Check engine light depending on the maker (Saab: none)

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–200
Time0.3–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

power steering pressure switchsensor wiringengine control module

Related codes

P0550P0552P0553P0554P0501P0506

References: mitsubishitechinfo.com, saabwisonline.com, saabwisonline.com, law.cornell.edu.