Powertrain, generic · Vehicle speed, idle control and auxiliary inputs
P05C9 Economy Mode Switch Circuit Low
Check engine lightOn
EmissionsNot relevant
Drive cycleRequired
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. 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.
Likely causes
Signal or reference line shorted to ground
Switch shorted internally
Shared 5 V reference loaded by another sensor
Module
Symptoms
- Eco mode not selectable
- Eco indicator not working
- Vehicle runs in normal mode
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.