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

P0596 Cruise Control Servo Control Circuit High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

Older engines without electronic throttle use a vacuum cruise servo on the throttle cable: a vacuum valve (closed when unpowered) opens the throttle with intake vacuum, a vent valve (open when unpowered) holds or releases it, and a dump valve fed through the brake switch vents it when braking. Where the engine module drives these valves, it checks each circuit and switches cruise off; CARB treats this as emission-neutral, usually without warning lamp. P0596 means the voltage in the servo supply or control circuit is too high, for example a short to positive keeping supply on while braking – safety-relevant, because the brake switch cut-off could then fail; cruise control is locked out. No manufacturer document available to us uses this code.

Likely causes

Servo supply line shorted to positive
Brake light switch sticking
Servo faulty
Engine module

Symptoms

  • Cruise control cannot be engaged or switches off
  • Cruise indicator goes out or flashes (depending on vehicle)
  • Usually no check engine light, code stored only

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–300
Time0.5–2 h

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

Parts involved

cruise control servobrake light switchservo wiringengine control module

Related codes

P0594P0595P0584P0588P0571

References: patents.google.com, patents.google.com, law.cornell.edu.