Powertrain, generic · Vehicle speed, idle control and auxiliary inputs

P0561 System Voltage Unstable

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

The module measures its own supply voltage. CARB allows other monitors to be suspended below 11 V, and above a maker's threshold only if the voltage itself is diagnosed – that is what these codes record. GM's engine modules and Ford's PCM set them without warning lamp. GM interprets P0561 as a difference between the module's two supply inputs: permanent battery and switched ignition supply may differ by at most 3 V, otherwise it sets after 50 of 63 samples. In the engine module that is type C without warning lamp, in GM's 9-speed transmission module type A with the lamp after one trip. Other makers use it for a fluctuating voltage.

Likely causes

Fuse, relay or line of the ignition supply (voltage drop)
Loose or corroded connector or ground at the module
Voltage regulator fluctuating
Battery terminal loose

Symptoms

  • Further voltage or communication codes
  • Transmission warning or check engine light (GM transmission module)
  • Sporadic dropouts of electrical functions

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–300
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

batteryalternatormodule power supplyengine control module

Related codes

P0560P0562P0563P0685

References: gsi.ext.gm.com, gsi.ext.gm.com, law.cornell.edu.