Powertrain, generic (extended)

P256D Engine Idle Speed Selector Sensor/Switch Circuit High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P256D is logged when the idle speed selector signal rises above the highest value the controller treats as valid. Causes include a wire chafed against a battery-fed circuit, an open sensor return that lets the input float high, or an accessory module that drives the line with system voltage. No manufacturer documents P256D publicly. On GM's 6.6L Duramax trucks covered by PIT4529D, the standard elevated idle lifts engine speed to 1050 rpm only below 0 °C outside and with coolant under 65 °C, and owners can switch it off; a selector fault should not be confused with that intended behaviour. P256C is the low-side counterpart and P256E the intermittent variant. Usually the raised idle cannot be selected, while normal driving is unaffected.

Likely causes

Selector wire chafed against a battery-fed circuit
Open return line letting the input float high
Accessory module feeding system voltage into the line

Symptoms

  • Raised idle cannot be selected
  • Normal driving unaffected

Repair estimate

DifficultyEasyDIY possible
Parts and labour€20–250
Time0.3–1.5 h

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

Parts involved

idle speed selector switch

Related codes

P256AP256BP256CP256EP0507

References: law.cornell.edu, static.nhtsa.gov.