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

P0594 Cruise Control Servo Control Circuit/Open

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. P0594 concerns the servo control or supply circuit: an open circuit. Where the servo is fed through the cruise contact of the brake light switch, braking interrupts this supply on purpose, so the brake switch and its wiring are typical causes and brake lights may be affected too. No manufacturer document available to us describes P0594 in detail.

Likely causes

Brake light switch (cruise contact) faulty or misadjusted
Supply line open or shorted to ground
Servo connector or servo
Engine module output

Symptoms

  • Cruise control cannot be engaged or switches off
  • Usually no check engine light, code stored only
  • Possibly brake light problem as well (shared switch)

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

P0595P0596P0582P0586P0571

References: patents.google.com, law.cornell.edu, en.wikipedia.org.