P0594 code meaning
Cruise Control Servo Control Circuit/Open
What P0594 means
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.
P0594 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 2 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0594
- Cruise control cannot be engaged or switches off
- Usually no check engine light, code stored only
- Possibly brake light problem as well (shared switch)
Common causes of P0594
- Most likelyBrake light switch (cruise contact) faulty or misadjusted
- PossibleSupply line open or shorted to ground
- PossibleServo connector or servo
- Less likelyEngine module output
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Cruise control servoBrake light switchServo wiringEngine control module
What each part of P0594 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 5 | Area: vehicle speed, idle control and auxiliary inputs. |
| 94 | The exact fault in that area. |
What to do next
- Note P0594 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0594
What does P0594 mean?
P0594 means "Cruise Control Servo Control Circuit/Open". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0594?
P0594 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P0594 fail an emissions test?
P0594 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P0594 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0594 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.