Why does a low reading mean the vent is stuck closed rather than open?

Because the module energises this solenoid by pulling its line low, not by feeding it. The vent coil is switched on its ground side, so low is the energised state — and the vent is the element that sits open when it has no power. A line found low when the module was not commanding it therefore describes a coil being held energised, which holds the vent shut. That is why this code points in the opposite direction from the open-circuit fault, where losing power vents the servo and releases the throttle exactly as the designers intended.

Will the throttle stay open until I switch the engine off?

No. A cruise servo is not sealed — it has a rubber diaphragm, a hose, a fitting and a linkage, and all of them leak slightly, so trapped vacuum bleeds away rather than holding. What drivers actually report is a throttle that hangs for a beat or a few seconds on release, not one that stays up. The brake pedal releases the servo mechanically on essentially every vehicle with this hardware, and the throttle return spring closes the throttle by itself, so the car stays controllable the whole time.

Two cars have the same fault but one is much worse. Why?

Because the hang time is set by how well the servo holds vacuum, not by how bad the electrical fault is. A servo with a tired diaphragm and hardened hoses bleeds down in under a second, so the driver barely notices; a tight one holds for several seconds and the complaint is obvious. This is worth using rather than puzzling over — apply a hand vacuum pump to the servo and watch the gauge, and the leak rate you measure explains the complaint you were given without any electrical testing at all.

Is this worth fixing straight away, or can it wait?

It is worth doing promptly, and for a specific reason rather than general caution. Most cruise control faults leave the system unable to act, which costs you a convenience feature. This one leaves it able to act in a way you did not ask for — a throttle that lags on release. The brakes handle it comfortably and nothing about the car is unsafe, but an unexpected hang in traffic is unpleasant enough that living with it is a poor trade against a repair that is usually a wire or a $40 solenoid.

OBD-II trouble code

P0587: Cruise Control Vent Control Circuit Low

This is the one fault in the vent block that pushes against the fail-safe rather than with it: a low reading means the vent is being held shut, so vacuum gets trapped in the servo instead of released. How long the throttle hangs before it lets go is a free measurement of how tight your servo is.

Medium severityPowertrainCruise ControlDrivable short-term

Quick facts

System: Powertrain
Category: Cruise Control
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $90 – $650
DIY difficulty: Intermediate DIY

What does P0587 mean?

P0587 means the module watched the control line for the cruise control vent solenoid and found it sitting at a lower voltage than it should have been. Reading that correctly requires knowing two things about the hardware — which side of the coil the module switches, and what the vent does when nothing is switching it at all — and once you have both, this code tells a very specific story.

On a vacuum cruise servo the module controls its solenoids on the ground side. It energises a coil by pulling that coil's line down toward ground, so on this circuit low is not a fault state in itself: low is the working state. What sets the code is the module finding the line low at a moment when it was not asking for that. Something other than the module's own output stage is completing the path to ground.

The consequence is where this code separates from everything else in the block. The vent is the element that lets atmospheric air back into the servo, and it is built to be open when de-energised — that is the deliberate fail-safe that means a loss of power releases the throttle. A vent held energised is a vent held shut. So this fault does not fail toward the safe state; it works against the design. Air cannot get back into the servo, the vacuum that was admitted to pull the throttle open stays there, and the throttle stays up after the driver has asked for it to come down.

It is worth being honest about how long that lasts, because the internet version of this code is more dramatic than the physics. A cruise servo is not a sealed vessel. It has a rubber diaphragm, a hose, a fitting and a mechanical linkage, and every one of those leaks a little. Trapped vacuum therefore bleeds away rather than holding indefinitely, and what the driver experiences is a throttle that lags on release — hanging for a beat, or a few seconds, before it comes down — rather than one that stays open until the ignition is switched off. The car remains controllable throughout. The brake pedal releases the servo mechanically on essentially every vehicle fitted with this hardware, and the throttle return spring does the rest.

That leak rate turns out to be diagnostically useful, which is unusual. How long the throttle hangs is a direct read on how tight the servo is. A driver who reports a lag of half a second has a servo that leaks freely; a driver who reports several seconds has one that holds well. So the severity of the complaint is telling you about the condition of the diaphragm and hoses, not about the severity of the electrical fault. Two vehicles with identical wiring faults will produce noticeably different complaints for that reason, and reading the complaint as a measure of the electrical problem will send you looking for a bigger short than exists.

The reason this code is rated a step above the other vent faults is not that the car becomes dangerous. It is that most cruise control faults leave the system unable to do anything, whereas this one leaves it able to do something the driver did not ask for. A throttle that lags on release is an unwelcome surprise in traffic even when the brakes deal with it comfortably, and it is worth fixing promptly rather than living with — which is a different judgement from the open-circuit code, where the only loss is a convenience feature.

Common causes

Vent solenoid control wire shorted to ground where the loom is pinched or chafed against metal
Vent solenoid winding broken down internally to the solenoid body
Corrosion inside the servo connector bridging the control terminal to the shell
Module low-side driver failed in the conducting state for the vent output
Damaged insulation where the servo loom crosses a bulkhead flange or bracket stud
Moisture tracking inside a servo connector that has lost its seal
Incorrectly repaired splice from earlier work in the engine bay bridging the vent line
Aftermarket wiring tapped into the cruise circuit and grounded to the body

Symptoms

Throttle lagging for a beat or several seconds after cruise control is cancelled
Speed continuing to build briefly after the driver asks the system to slow
Cruise control holding speed poorly on the low side of the set point
Needing the brake pedal to bring the throttle down after cancelling
Cruise control disabling itself after a single use
Check engine light with no change in how the engine runs
Symptom severity varying between otherwise identical vehicles
Complaint appearing after engine bay work near the servo loom

Diagnostic steps

1.Ask the driver about timing before you open the hood, because on this code the complaint is a clock reading and not a symptom list. What matters is how long the throttle stayed up after they cancelled: a fraction of a second is normal, one or two seconds is a hint, and anything the driver describes as needing the brake pedal to overcome is a definite finding. Write the number down, because you will use it twice.
2.Establish which way this circuit is driven before interpreting the word low. On these servos the module switches the solenoid on its ground side, which means pulling the line low is how the module energises the vent — and the vent is the element that is open when it has no power. So a line found low that the module did not command low describes a vent held shut. That is the opposite direction from the fail-safe the hardware was designed around, and it is the whole reason this code reads differently from P0586.
3.Unplug the vent solenoid and read the line again with the ignition on. If the line stays low with the coil disconnected, the low path is in the harness or in the module's output stage and the solenoid is a bystander. If the reading rises to something sensible the moment the coil comes off, the low path runs through the winding, which points at a coil that has broken down internally to its own housing.
4.Measure the vent coil's resistance to its case, not just end to end. A winding that has chafed through its varnish onto the solenoid body reads a perfectly normal few tens of ohms across the terminals and still pulls the control line down, so an end-to-end check on its own will clear a part that is faulty. This is the measurement that finds the fault the resistance check misses.
5.Perform a bleed-down test on the servo with a hand vacuum pump while you are there, since the trapped-vacuum symptom depends entirely on how well the servo holds. Apply vacuum to the servo port and watch the gauge. A diaphragm and hose set in good condition holds for tens of seconds. If the gauge collapses in two or three, the servo leaks — and the driver's throttle-hang time will be short for that reason rather than because the electrical fault is mild. Read the two findings together, not separately.
6.Trace the vent circuit's ground-side conductor for contact with metal rather than for breaks. A low reading is created by an unwanted connection, so the inspection target is different from an open-circuit code: look at points where the loom is pinched under a bracket, run along a bulkhead flange, or passes a stud, and look for insulation that has been polished shiny rather than for wire that is visibly broken.
7.Confirm the repair on the road with the timing figure from step one, not with a cleared dashboard. Set cruise at a steady speed, cancel with the stalk rather than the brake, and count. A throttle that drops immediately means the vent is releasing as designed. A throttle that still hangs, with no code returning, means you fixed the electrical fault and left the pneumatic one behind.

Repair cost

$90 – $650

Diagnosis runs $90 to $180, a little above the open-circuit code because finding a short to ground means tracing a conductor rather than reading a coil. Repairing a chafed wire or cleaning a corroded connector is $70 to $220 and is the most common outcome. A separately available vent solenoid is $30 to $150 plus half an hour to an hour. Where the solenoid is integral, a complete servo is $120 to $600 plus one to three hours including linkage adjustment. Budget separately if the bleed-down test shows a leaking diaphragm, because that is a second fault and the electrical repair will not address the lag it causes.

Estimate your repair

Run the numbers for your vehicle

Open the Repair Cost Estimator with cruise control servo / actuator replacement preselected. Adjust labor rate and vehicle category to fit your situation.

Estimate your repair cost

DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.