Comprehensive diagnostic guide for OBD-II code P0585 Cruise control related

Necessity and scope

• P0585 is a powertrain-related code in the OBD-II framework. Based on typical automotive practice, P0585 is commonly associated with a cruise control input/release relay circuit fault. However, this exact meaning is not explicitly defined , so treat the cruise control relay/input circuit interpretation as the likely area to inspect, while confirming with vehicle-specific wiring diagrams during repair.

Symptoms

• Cruise control will not engage or will disengage unexpectedly.
• Cruise control indicator on the dash may stay on or flash; or the vehicle fails to enter a cruise state when commanded.
• Brake or clutch input may not correctly disable cruise control, or the resume function may be unreliable.
• Intermittent cruise control operation or a sudden loss of function without other engine performance issues.
  Note: The above symptom set aligns with cruise control circuit faults typical for P0xxx powertrain codes, as these systems interact with the brake/clutch inputs and relay circuitry. Specific symptom patterns can vary by vehicle make/model.

Probable Causes

• Cruise control relay circuit faults and associated wiring (high probability: 40-70%)
  • Faulty relay (coil or contacts)
  • Damaged or corroded wiring harness to the relay or to the cruise control module
  • Poor or loose connectors in the relay circuit
• Brake/clutch input circuit issues (moderate probability: 15-40%)
  • Faulty brake pedal switch (or its wiring/connector)
  • Faulty clutch pedal switch (if equipped)
  • Incorrect brake switch adjustment or a short to ground
• Cruise control module or associated control wiring (low to moderate probability: 5-20%)
  • Faulty cruise control module (CCM) or its internal circuits
  • CAN/though- or data-bus communication issues affecting cruise control logic (if the vehicle uses networked control)
• Power supply or fusing issues (low probability: 5-15%)
  • Blown fuse for the cruise control circuit
  • Power supply faults to the CCM or relay coil
• Other related powertrain control issues (low probability: 0-10%)
  • Intermittent PCM/ECU issues that affect cruise control enable/disable logic
    Note: The above percentages are provided as practical field guidance in the absence of NHTSA complaint data . Treat them as general anchors rather than precise statistics.

Safety Considerations

• Cruise control systems involve speed control; work with ignition OFF and the vehicle secured during initial inspections of wiring, relays, and switches.
• When performing electrical tests, isolate the battery as needed and re-connect carefully to avoid short circuits.
• If the vehicle's brake or clutch switch wiring is suspect, verify operation before road testing to avoid unintended acceleration or loss of control.

Diagnostic Approach

1) Initial verification

• Confirm P0585 is the active DTC and note any freeze-frame data or related codes.
• Check for other DTCs that may point to the cruise control system (e.g., cruise-related codes, brake switch, or CAN/bus codes).
• Use a scan tool to read current and historical data for cruise control status, brake switch status, and any relay network activity.

2) Visual and mechanical inspection

• Inspect the cruise control relay and its power/ground supply wiring for corrosion, damage, or loose connections.
• Inspect the wiring harnesses and connectors for the cruise control module, relay, brake switch, and clutch switch (if present). Look for pin corrosion, frayed insulation, and bent terminals.
• Inspect brake pedal switch and clutch switch (if equipped) for proper actuation and alignment. Verify that the switch changes state correctly when pedals are pressed/released.

3) Electrical measurements and functional checks

Power and ground

• With ignition ON (engine off), use a multimeter to verify there is battery voltage at the relay coil feed when cruise control is enabled/disabled as per vehicle procedure.
• Verify a solid ground reference on the relay and CCM circuits.

Brake and clutch input circuits

• Check the brake pedal switch circuit for proper operation: apply and release the brake pedal and verify the switch transitions between ground and voltage as designed.
• For manual transmission vehicles, verify the clutch pedal switch behaves similarly and that the ECU/CCM recognizes the clutch state correctly.
• Look for short-to-ground or short-to-battery conditions on brake/clutch input circuits.

Cruise control relay operation

• With the ignition on, listen for the relay click when the cruise control is commanded ON in the scan tool. If no click or intermittent click occurs, suspect the relay, the control wiring, or the CCM.
• If your vehicle uses an integrated CCM or CAN bus architecture, verify the CCM is receiving the proper "enable" signals from the brake/clutch switches and from the vehicle speed sensor.

Module and wiring diagnostics

• Inspect the cruise control module (or integrated CCM) for signs of water intrusion, overheating, or corrosion on connectors.
• If equipped with a CAN bus or networked control, verify there are no related network faults that could affect cruise control logic.

4) Functional tests and re-checks

• Clear DTCs and perform a controlled road test to attempt to engage cruise control at a steady, safe speed; observe if the code returns and under what conditions.
• If the code reappears, attempt to reproduce the fault by cycling cruise control ON/OFF, engaging/disengaging via brake/clutch, and by simulating fault conditions (e.g., unplugging the brake switch while stationary). Do not perform risky unduly tests; follow vehicle manufacturer guidelines.
• Recheck for any new DTCs after testing.

5) Repair recommendations (order of operations)

• If a faulty relay is identified (visibly damaged, not energizing, or showing excessive contact wear), replace the cruise control relay and any affected wiring harness sections.
• Repair or replace damaged wiring or connectors in the cruise control circuit; use dielectric/terminal protectant as appropriate.
• If brake/clutch switch is tested faulty or out of specification, replace the switch and re-test cruise control operation afterward.
• If the CCM is suspected (electronic fault or network issue), consider a CCM replacement or re-programming/flash as per vehicle service information; verify with the OEM procedure.
• After repairs, re-scan and clear codes; perform a road test to confirm proper function and absence of recurring DTCs.

6) Verification and road test

• Confirm that the cruise control can be engaged and maintained at set speed, remains engaged through gentle slope changes, and disengages correctly when braking or pressing the clutch (as applicable).
• Re-scan to ensure there are no lingering or new DTCs.

7) Documentation and follow-up

• Record all diagnostic findings, parts replaced, and tests performed.
• Note any reprogramming or software updates if applicable.
• If the fault reoccurs, re-evaluate for a wiring harness harness-to-module fault, harness routing issues (exposure to heat, abrasion), or a latent PCM/ECU issue.

Tools Needed

• OBD-II scan tool capable of reading live data and DTCs, with freeze-frame data and CAN bus diagnostics if applicable.
• Digital multimeter (DVOM) for voltage, resistance, and continuity checks.
• Electrical test leads, current clamp (optional) for current draw on the cruise control relay circuit.
• Service information for your specific vehicle (wiring diagrams, connector pinouts, and relay specifications).
• Optional: Diagnostic scope/lab scope for reading sensor waveforms if cruise control logic is complex or uses PWM-type control.

Summary

• P0585 is a powertrain-related DTC that, in typical practice, points toward cruise control input/relay circuit faults. do not define P0585 explicitly, so approach with a focus on cruise control relay, brake/clutch inputs, and CCM wiring as primary suspects.

• A structured approach-confirm code, verify related circuits, inspect wiring/connectors, test the relay and control modules, perform functional tests, and verify with a road test-provides the most reliable path to resolution.

• Always verify in vehicle-specific service documentation, as wiring and control logic vary by make/model.

• OBD2 CODE DEFINITIONS. The specific P0585 definition is not provided in the excerpt; no explicit code description available in the given GitHub entry. )

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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