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DTC codes / GM codes / P1558

P1558 · GMCruise Control Servo Indicates Low

P1558 on a GM vehicle means the powertrain control module (PCM) has detected that the cruise control servo signal circuit is reporting a low voltage condition, indicating the servo is not receiving the expected power or feedback. The PCM monitors the servo's electrical circuit and sets this code when the voltage on the servo signal line falls below the calibrated threshold, which typically prevents the cruise control system from engaging or maintaining speed.

SystemPowertrain
Code typeGM specific
SeverityLow
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • The cruise control will not engage or will disengage unexpectedly while driving.
  • The cruise control indicator light may flash or remain off when you attempt to set a speed.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.
  • The engine may run normally with no drivability issues, as this code is isolated to the cruise control system.
  • In some cases, the cruise control may work intermittently, engaging only on certain trips or after the vehicle warms up.

Common causes, most likely first

  1. A faulty cruise control servo, where the internal electronics or motor have failed and are drawing low voltage.
  2. Damaged or corroded wiring in the servo signal circuit, including open circuits or high resistance connections.
  3. A poor ground connection at the servo or chassis, causing a voltage drop that the PCM interprets as low servo signal.
  4. A blown fuse or faulty relay in the cruise control power supply circuit, though this often affects multiple functions.
  5. A malfunctioning brake pedal position sensor or clutch switch (on manual transmissions) that interrupts the cruise circuit and sets a low-voltage condition.
  6. Corrosion or water intrusion at the servo connector, leading to intermittent low signal voltage.
  7. Rarely, a PCM internal fault misinterpreting the servo signal, but this should only be considered after all other causes are ruled out.

How to diagnose and fix it

  1. Inspect the cruise control servo connector and wiring for corrosion, loose pins, or chafed insulation, and repair as needed.
  2. Check the cruise control fuse and relay in the underhood fuse block, replacing any blown fuse or faulty relay.
  3. Measure the voltage and ground at the servo connector with the key on and cruise control commanded on, comparing to factory specifications.
  4. Test the servo motor and clutch for proper resistance and operation using a scan tool or bidirectional controls if available.
  5. Verify the brake pedal position sensor and clutch switch operation, as they can interrupt the servo circuit and cause a low-voltage code.
  6. If voltage and ground are correct but the servo does not respond, replace the cruise control servo with a known-good unit.
  7. Clear the code and perform a road test to confirm the cruise control engages and the code does not return.

What a low servo signal really means

  • A low servo signal does not automatically mean the servo is bad; a voltage drop on the ground side can mimic a failed servo and is often overlooked.
  • The brake pedal position sensor is a common culprit because it directly interrupts the cruise control circuit, and a faulty sensor can set P1558 without any brake-related symptoms.
  • Intermittent P1558 codes often appear after exposure to moisture or road salt, so inspect the servo connector and harness routing for water intrusion before replacing parts.
  • On some GM vehicles, the cruise control servo shares a ground with other body components, so a corroded ground stud can cause this code along with other electrical gremlins.
  • A scan tool that can command the cruise control servo on and off is invaluable for confirming whether the fault is in the servo itself or in the wiring and switches.

Can I keep driving with P1558?

The vehicle remains safe to drive because the cruise control system is disabled, but you should avoid using cruise control until the fault is repaired. The code does not affect engine performance or braking, so normal driving can continue while you schedule diagnosis.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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