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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P05FA

P05FAStop-Start Status Signal Circuit Low

P05FA indicates that the engine control module has detected a low voltage condition on the stop-start status signal circuit, which is used to monitor the readiness of the vehicle's automatic stop-start system. The circuit voltage has fallen below the manufacturer's expected threshold, preventing the system from engaging or causing it to be disabled.

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

Symptoms you may notice

  • The stop-start system does not activate, so the engine never shuts off at stops.
  • A warning light or message on the dashboard indicating stop-start is unavailable.
  • The engine may hesitate or stall when coming to a stop if the system attempts to engage.
  • Often no symptoms besides the warning light, with the vehicle driving normally otherwise.
  • In some cases, other electrical functions like interior lights or accessories may flicker.

Common causes, most likely first

  1. A faulty stop-start status sensor or switch that is stuck open or has high resistance.
  2. Damaged or corroded wiring in the stop-start status signal circuit, including chafed insulation or broken strands.
  3. Loose or corroded electrical connectors at the sensor or the control module.
  4. A weak or failing vehicle battery that cannot maintain proper voltage for the stop-start system.
  5. A defective engine control module or stop-start control module, though this is less common.
  6. Poor ground connection for the stop-start system components.

How to diagnose and fix it

  1. Inspect the battery state of charge and perform a load test; recharge or replace if it fails.
  2. Check all fuses and relays related to the stop-start system and replace any that are blown.
  3. Visually inspect the stop-start status sensor and its wiring for damage, corrosion, or loose connections; repair or replace as needed.
  4. Use a multimeter to measure voltage on the signal circuit at the sensor and at the control module; compare to manufacturer specifications.
  5. Perform a voltage drop test on the ground circuit to ensure a good ground path.
  6. If the circuit tests good, test the stop-start status sensor with a scan tool to verify its output; replace if faulty.
  7. Clear the diagnostic trouble codes and verify the repair by test-driving the vehicle and confirming the stop-start system operates correctly.

Stop-Start Signal Circuit Low: Hidden Causes

  • A weak battery is often the root cause of this code, even if the engine starts normally, because the stop-start system requires a higher state of charge to engage.
  • The stop-start status signal may be shared with other systems, so a fault in a related module can set this code without any issue in the stop-start components themselves.
  • Corrosion inside a connector can cause a low signal without any visible damage, so always perform a wiggle test and inspect terminals closely.
  • After replacing a component, the stop-start system may require a relearn procedure or calibration with a scan tool to function correctly.
  • This code can be set by a momentary voltage drop during engine cranking, so check for a weak battery or poor connections before replacing parts.

Can I keep driving with P05FA?

The vehicle remains safe to drive because the stop-start system is disabled, but avoid relying on the feature and address the issue promptly to prevent battery drain or other electrical problems. Do not attempt to force the stop-start system to engage.

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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