Code
P05C9
Generic
P — Powertrain
Economy Mode Switch Circuit Low
AI status
Completed
Completed
100%
Causes
- Faulty economy mode switch (internal short or open)
- Short to ground in the switch wiring or connector
- Broken or corroded connector terminals
- Open circuit (broken wire) between switch and ECU
- Blown or poor fuse/power/ground connection for the switch circuit
- ECU/BCM input fault or damaged ECU pin
Symptoms
- MIL/Check Engine Light (DTC stored)
- Economy/Drive mode selection not working or not responding
- Instrument cluster or HVAC display may not show mode change
- Intermittent operation of economy mode switch
- Possible degraded fuel economy if mode defaults to non-economy
What to check
- Retrieve freeze frame and all stored codes with a scan tool; note related codes.
- Confirm battery voltage is within normal range (12.4–14.6 V) before testing.
- Visually inspect the economy mode switch, mounting, and connector for damage, corrosion, or water ingress.
- Check fuses and fusible links related to the switch/ECU circuits.
- Backprobe the switch connector and check signal voltage while operating the switch.
- Perform a wiggle test of wiring while observing live data for intermittent changes.
Signal parameters
- Signal type: digital on/off switch input to ECU (binary state).
- Typical low voltage (logic LOW): 0.0–1.0 V (indicates circuit pulled to ground).
- Typical high voltage (logic HIGH): ~4.0–5.0 V for 5V reference systems, or ~11–14 V in some 12V-switched circuits; check vehicle-specific wiring diagram.
Diagnostic algorithm
- Connect a scan tool and confirm P05C9 is current or stored; record freeze frame data and related codes.
- Verify battery voltage and ground integrity. Low system voltage can cause false readings.
- Perform a visual inspection of the switch, connector, and harness for damage, corrosion, or water entry. Repair as needed.
- Locate the economics mode switch wiring diagram or pin-out. Identify the signal, power (if applicable) and ground pins.
- Backprobe the switch connector. With the ignition ON (engine off), operate the switch while watching live data or measuring voltage: verify the signal toggles between expected LOW and HIGH values.
- If the signal stays low, check for a short to ground on the signal wire (measure resistance to ground with connector disconnected).
- Check continuity between the switch signal pin and the ECU input pin. Repair any open or high-resistance wiring.
- Inspect and test any pull-up resistor or reference feed in the circuit (some designs pull the input high via ECU). Replace the switch if wiring and pull-up are good but input does not change.
- If wiring and switch check good, suspect ECU/BCM input fault — verify power/grounds to ECU and consider ECU bench test or replacement per manufacturer guidance.
- After repairs, clear codes, perform relearn procedures if required, and verify proper operation and that DTC does not return.
Likely causes
- Corroded/loose connector at the switch
- Damaged or chafed wire shorting to ground
- Failed economy mode switch (internal failure)
Fault status
Status
Economy Mode Switch Circuit Low - input below expected threshold
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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