What it means
Same as P05C7 with the signal shorted to ground. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
How it works
The economy-mode switch sends the driver's ECO request to the control module over a signal circuit. P05C9 sets when the signal is lower than expected, commonly because it is shorted to ground or the connector has a poor connection.
Common causes
Most likely first.
- Signal shorted to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
Symptoms
- Check-engine light
- Economy mode may not respond to the switch
- Economy mode may be unavailable or remain selected
How to diagnose it
- Save freeze-frame data and check live ECO switch status to see whether the input remains active or fails to change when the button is operated.
- Inspect the switch connector and harness for chafing, exposed wire touching ground, moisture, corrosion, or pushed-back pins.
- Move the harness gently while watching switch status to check whether the low input changes.
- Using the wiring diagram, test the signal circuit for a short to ground and check connector terminals for high resistance or poor contact.
- If the circuit is sound, compare switch output with the manufacturer's specification and replace the switch only if it fails.
Common fixes
- Repair insulation where the signal wire is chafed or contacting ground.
- Clean and secure corroded or loose switch terminals.
- Replace a damaged connector or terminal that cannot maintain contact.
- Replace the economy-mode switch if its signal remains low with a verified circuit.
Can I keep driving?
Driving is usually possible if the vehicle otherwise operates normally, but the ECO switch may not change modes or the module may interpret the input incorrectly. Watch for unexpected changes in throttle response, shifting, or climate operation.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.