Code
P05C8
Generic
P — Powertrain
Economy Mode Switch Circuit Range/Performance
Views:
UK: 25
EN: 42
RU: 33
AI status
Completed
Completed
100%
Causes
- Open or short in the Economy Mode switch wiring
- Corroded, loose or damaged connector at the switch or ECU
- Failed Economy Mode switch (mechanical or electrical)
- Short to battery voltage or short to ground on the signal circuit
- Intermittent contact (wiring chafe, pinched harness) or water intrusion
- Faulty ECU/PCM input circuit or internal software fault
Symptoms
- Malfunction Indicator Lamp (MIL) or dash warning light illuminated
- Economy Mode may not engage or may be stuck on
- Erratic or inconsistent activation of Economy Mode
- Possible change in shift strategy or engine response when economy mode is selected
- Reduced fuel economy if mode is stuck off or incorrect behavior
What to check
- Retrieve freeze frame and live data with a scan tool; note Economy Mode switch state and related inputs
- Check for additional DTCs that may indicate wiring or ECU faults
- Visually inspect switch, connector, and wiring harness for damage, corrosion or water entry
- Backprobe the switch connector and ECU input to measure signal voltage with the switch toggled
- Perform a wiggle test on wiring while observing live data for intermittent faults
- Compare measured voltages/resistance to specifications from the wiring diagram/service manual
Signal parameters
- Typical switch signal: low ~0.0–0.5 V (OFF) and high ~4.5–5.5 V (ON) for a pull‑up to reference voltage
- Some systems use ground-switched circuits: 0 V = ON, open/high impedance = OFF
- Expected steady, debounced binary signal when toggling the switch (no rapid fluctuation)
- Intermittent or out-of-range voltages indicate wiring, connector, or switch fault
Diagnostic algorithm
- Connect a scan tool and confirm P05C8 is current; record freeze frame and related codes.
- Observe live data and toggle the Economy Mode switch while monitoring the input signal/state.
- Visually inspect the switch, connector, and harness for damage, corrosion, or loose terminals.
- Backprobe the switch connector and measure voltage/continuity: verify proper reference voltage and ground.
- If the switch is a simple contact, measure resistance across the switch while operating it to confirm open/closed states.
- Trace the wiring from the switch to the ECU; check for continuity, short to voltage, and short to ground.
- Repair any wiring or connector faults (repair splices, replace damaged harness sections, clean/replace connectors).
- If wiring and connector tests pass, substitute or bench-test the switch (or replace) and retest.
- If fault persists with known-good switch, test or consult procedures for the ECU input circuit; repair or replace ECU only after confirming input circuit failure.
- After repairs clear DTCs, perform a road test and re-scan to confirm the code does not return.
Likely causes
- Broken/damaged wire in harness to the switch
- Connector terminal corrosion or poor mating at switch or ECU
- Failed or stuck Economy Mode switch on the dash/console
- Aftermarket accessories tied into the same circuit
- ECU input damaged by over-voltage or previous short
Fault status
Status
P05C8 — Economy Mode Switch Circuit Range/Performance: Input signal to the control module is outside the expected range or not performing correctly.
Repair difficulty: Medium
Diagnostic time: 0.5-2.5 hours
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