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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P05C9

P05C9Economy Mode Switch Circuit Low

P05C9 indicates that the powertrain control module has detected a low-voltage condition on the economy mode switch circuit, meaning the signal wire is reading near ground when it should be at a higher reference voltage. The economy mode switch, often part of a driver-selectable driving mode system, allows the driver to toggle between economy and normal performance settings, and this code sets when the circuit is shorted to ground or the switch is stuck closed.

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

Symptoms you may notice

  • The check engine light or service vehicle soon message illuminates on the dashboard.
  • The economy mode indicator may fail to light up or may stay illuminated constantly regardless of switch position.
  • The vehicle may remain stuck in economy mode, resulting in sluggish acceleration and reduced throttle response.
  • The driver may find that pressing the economy mode button produces no change in vehicle behavior.
  • Often no symptoms besides the warning light, especially if the vehicle defaults to a safe operating mode.
  • In some vehicles, other driving modes such as sport or snow may also become unavailable.

Common causes, most likely first

  1. A faulty economy mode switch that is internally shorted to ground, keeping the circuit at low voltage.
  2. A chafed or pinched signal wire between the switch and the control module shorting to chassis ground.
  3. Corrosion or water intrusion in the switch connector or inline harness connectors, creating a low-resistance path to ground.
  4. A defective control module driver circuit that incorrectly reads the switch input as low.
  5. Aftermarket accessories or wiring repairs that tapped into the switch circuit and introduced a ground short.
  6. A stuck or damaged button mechanism that physically holds the switch in the closed position.

How to diagnose and fix it

  1. Connect a scan tool and verify P05C9 is current, then check the economy mode switch live data to see if the status changes when the button is pressed.
  2. Inspect the switch connector and harness for corrosion, water intrusion, or loose pins, and clean or repair as needed.
  3. Disconnect the switch and measure resistance across its terminals in both pressed and released states to confirm it opens and closes correctly.
  4. With the switch disconnected, measure voltage on the signal wire from the control module to verify it is near the expected reference voltage (often 5 volts) and not shorted to ground.
  5. If voltage is low with the switch disconnected, trace the harness for a short to ground, checking areas near sharp edges, hinges, or aftermarket installations.
  6. If the circuit tests good, substitute a known-good switch or use a jumper wire to simulate the switch and observe scan tool data for proper state changes.
  7. If all external circuits and the switch are verified, suspect the control module and perform further diagnostics before replacement.

Don't overlook the switch connector

  • A common cause of this code is water intrusion into the switch connector, which creates a partial short to ground that mimics a failed switch but is fixed by cleaning and sealing the connector.
  • The economy mode switch is often part of a multi-function button cluster, so a single failed button can set this code while other buttons on the same panel continue to work normally.
  • Aftermarket remote starters or alarm systems that tap into the mode switch wiring are a frequent source of unintended ground shorts on this circuit.
  • Some vehicles will default to economy mode when the switch circuit fails low, so the driver may not notice any performance change and assume the code is harmless.
  • A scan tool that can read switch input status is far more reliable than a voltmeter alone, because the control module may use a pulsed signal that a basic meter cannot interpret correctly.

Can I keep driving with P05C9?

The vehicle remains safe to drive because the economy mode switch only affects driving mode selection and does not impact engine safety or emissions compliance. Avoid pressing the economy mode button repeatedly, as this can worsen a intermittent short, and have the circuit inspected at your next service visit.

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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