Powertrain, generic · Vehicle speed, idle control and auxiliary inputs
P05C8 Economy Mode Switch Circuit Range/Performance
Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo
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. P05C8 means the signal is in range but implausible – between valid ladder steps or permanently pressed; GM accepts a press only for 0.094–1.5 s.
Likely causes
Switch stuck (permanently pressed)
Contact resistance or corrosion shifting the voltage step
Software
Module input
Symptoms
- Eco mode switching unintentionally or not at all
- Indicator flickering or off
- Usually no check engine light
Repair estimate
DifficultyEasyDIY possible
Parts and labour€30–200
Time0.3–1.5 h
Typical range across vehicles; the real cost depends on what the testing finds.
Parts involved
economy mode switchswitch wiringengine control module
Related codes
References: saabwisonline.com, law.cornell.edu.