P05C9 code meaning
Economy Mode Switch Circuit Low
Powertrain system Generic code P05xx codes Check engine light
What P05C9 means
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.
P05C9 at a glance
- Check engine lightYes, usually on
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P05C9
- Eco mode not selectable
- Eco indicator not working
- Vehicle runs in normal mode
Common causes of P05C9
- Most likelySignal or reference line shorted to ground
- PossibleSwitch shorted internally
- Less likelyShared 5 V reference loaded by another sensor
- Less likelyModule
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Economy mode switchSwitch wiringEngine control module
What each part of P05C9 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 5 | Area: vehicle speed, idle control and auxiliary inputs. |
| C9 | The exact fault in that area. |
What to do next
- Note P05C9 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P05C9
What does P05C9 mean?
P05C9 means "Economy Mode Switch Circuit Low". The economy mode switch selects a fuel-saving mode (flatter pedal map, earlier upshifts, sometimes reduced A/C). It is a generic code, so it means the same on every make.
Is it safe to drive with P05C9?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P05C9 fail an emissions test?
P05C9 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P05C9 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P05C9 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.