P05C9Economy Mode Switch Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Needs a drive cycle to confirm a fix
- How the car detects it
- The economy mode switch selects a fuel-saving mode (flatter pedal map, earlier upshifts, sometimes reduced A/C). It is usually a resistor ladder with 5 V reference read by the engine module; CARB names a driver-selectable eco mode as an off-cycle technology whose use must be counted. GM reports the same switch with its own codes P159F/P15A0, type B with warning lamp after two trips; P05C7–P05CA appear in no manufacturer document available to us. P05C9 means the switch circuit voltage is too low – signal or reference shorted to ground; GM's equivalent P159F detects the same fault on the same part.
- What you may notice
- Eco mode not selectable
- Eco indicator not working
- Vehicle runs in normal mode
- Likely causes
- LikelySignal or reference line shorted to ground
- PossibleSwitch shorted internally
- RareShared 5 V reference loaded by another sensor
- RareModule
- Repair
- Easy · Often possible to do yourself.Typical cost: €30 – €200
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.