Powertrain, generic (extended)

P256A Engine Idle Speed Selector Sensor/Switch Circuit/Open

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P256A refers to the switch or sensor with which the driver chooses a raised idle speed, for example to run a generator, an inverter or belt-driven equipment while parked, independent of any PTO. The controller sees no valid state on this input, which points to an open wire, a disconnected plug or a switch that no longer makes contact. No manufacturer documents P256A publicly. GM's bulletin PIT4529D for 2007–2018 Silverado/Sierra 2500/3500 shows that such selections exist side by side: an automatic cold-weather elevated idle on the 6.6L diesel, an optional High Idle (RPO UF3) operated through the cruise buttons, and a separate PTO mode. The engine keeps its normal idle; P0506 and P0507 instead concern an idle that misses its target.

Likely causes

Open wire in the idle selector circuit
Selector connector loose or not mated
Idle selector switch without contact

Symptoms

  • Raised idle cannot be selected
  • Engine stays at normal idle

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 switchwiring harness

Related codes

P256BP256CP256DP256EP0506P0507

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