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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P05C8

P05C8Economy Mode Switch Circuit Range/Performance

P05C8 indicates that the powertrain control module (PCM) has detected an out-of-range or performance fault in the economy mode switch circuit, which is used to select a fuel-saving driving strategy. The computer compares the switch signal against expected values and sets this code when the voltage or switch state does not match the commanded mode within the calibrated time.

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

Symptoms you may notice

  • The economy mode indicator light may fail to illuminate or may stay on continuously.
  • The vehicle may remain in a default driving mode and not respond to switch presses.
  • A noticeable decrease in fuel economy if the vehicle is stuck in a non-economy mode.
  • The switch may feel unresponsive or require multiple presses to change modes.
  • Often no symptoms besides the warning light, especially if the fault is intermittent.

Common causes, most likely first

  1. Faulty economy mode switch, such as worn contacts or internal failure.
  2. Damaged or corroded wiring harness or connector at the switch or PCM.
  3. Open or short circuit in the signal or ground circuit for the economy mode switch.
  4. PCM software calibration issue or internal driver fault.
  5. Water intrusion or contamination in the switch or connector causing intermittent signals.
  6. Aftermarket accessories or wiring modifications interfering with the circuit.

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related codes in other modules.
  2. Inspect the economy mode switch and its connector for physical damage, corrosion, or loose pins.
  3. Test the switch operation using a multimeter to confirm continuity or voltage changes when pressed.
  4. Check the wiring harness for continuity, shorts to ground, or shorts to power between the switch and PCM.
  5. If the switch and wiring test good, check for technical service bulletins or software updates for the PCM.
  6. Replace the economy mode switch if it fails testing, then clear codes and verify the repair.
  7. If the fault persists, perform a pinpoint test for the PCM and replace if necessary.

Before you replace the economy mode switch

  • A common misdiagnosis is replacing the switch when the real problem is a chafed wire in the harness near the pedal or center console.
  • The economy mode switch often shares a ground with other dashboard switches, so a bad ground can set this code along with others.
  • In some vehicles, the economy mode is controlled through the body control module (BCM) rather than directly by the PCM, so check all modules for related codes.
  • Intermittent switch operation may not set the code until the vehicle is driven in specific conditions, so a road test with live data is essential.
  • Aftermarket remote starters or alarm systems can tap into the switch wiring and cause range/performance faults.

Can I keep driving with P05C8?

The vehicle remains safe to drive because the economy mode only affects fuel-saving strategy, not core engine or transmission operation. Avoid toggling the switch repeatedly and have the circuit inspected at your convenience to restore normal mode selection.

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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