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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P05F9

P05F9Stop-Start Status Signal Circuit

P05F9 indicates that the powertrain control module (PCM) has detected a fault in the stop-start status signal circuit, which is used to monitor the readiness of the automatic engine stop-start system. The PCM expects a specific voltage or signal pattern from the stop-start status sensor or switch, and when the signal is out of range, intermittent, or missing, it sets this code and may disable the stop-start feature.

SystemPowertrain
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • The stop-start system may be disabled, so the engine no longer shuts off at traffic lights or when idling.
  • A warning light or message on the dashboard indicating stop-start is unavailable.
  • The engine may hesitate or stumble when restarting from a stop, or the restart may be delayed.
  • In some cases, the vehicle may stall when coming to a stop if the stop-start system engages unexpectedly.
  • Often no symptoms besides the warning light, as the vehicle remains driveable without stop-start functionality.

Common causes, most likely first

  1. Faulty stop-start status sensor or switch, such as a worn or contaminated switch in the center console or dashboard.
  2. Damaged or corroded wiring harness or connector at the stop-start status sensor or PCM.
  3. Low or weak battery voltage, which can cause the stop-start system to set a status signal fault.
  4. Failed or failing stop-start auxiliary battery (if equipped) that cannot maintain proper voltage during engine-off events.
  5. Software glitch or outdated PCM calibration affecting stop-start signal processing.
  6. PCM internal fault, though this is less common and should be diagnosed last.

How to diagnose and fix it

  1. Verify battery condition and state of charge, including the auxiliary battery if equipped, and recharge or replace as needed.
  2. Inspect the stop-start status sensor and its connector for physical damage, corrosion, or loose pins; clean or repair as necessary.
  3. Check wiring continuity and voltage drop between the stop-start status sensor and the PCM, looking for opens, shorts, or high resistance.
  4. Use a scan tool to monitor live data for the stop-start status signal while operating the switch to confirm proper signal changes.
  5. If the sensor is suspect, test it according to the manufacturer's procedure and replace it if it fails the test.
  6. Check for technical service bulletins (TSBs) or software updates for the PCM related to stop-start system faults and reprogram if applicable.
  7. If all external circuits and components test good, follow the diagnostic flow chart for PCM replacement or repair, ensuring proper relearn procedures are performed.

Stop-Start Signal Faults You Might Overlook

  • A weak or aging battery is a leading cause of stop-start status signal codes, even when the engine starts normally, because the system requires stable voltage during engine-off periods.
  • The stop-start status signal may be derived from a simple switch or a more complex sensor integrated into the brake pedal or shifter, so misdiagnosis often occurs by replacing the wrong component.
  • Intermittent faults can be caused by a chafed wire that only shorts when the steering wheel is turned or the console is bumped, so wiggle tests are essential.
  • Some vehicles use a separate stop-start auxiliary battery that can fail independently of the main battery, and a failing auxiliary battery may not illuminate a traditional battery light.
  • The PCM may set P05F9 as a consequence of another fault, such as a crankshaft position sensor or throttle body issue, so always check for related codes before replacing parts.

Can I keep driving with P05F9?

The vehicle remains safe to drive because the stop-start system is a fuel-saving feature and its failure does not affect normal braking or steering. However, avoid relying on the stop-start function, and address the fault promptly to prevent battery drain or unexpected stalling in traffic.

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 all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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