Powertrain, generic (extended)

P256B Engine Idle Speed Selector Sensor/Switch Range/Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P256B means the idle speed selector delivers a value that is electrically valid but makes no sense in context. The controller may, for instance, see a raised-idle request with the truck in motion, a position that does not exist, or a request that conflicts with an active PTO mode; Ram's 2012 chassis cab guide states that remote PTO outranks the Idle Up function, so the two inputs have a defined order the software can check. CARB defines a rationality diagnostic as comparing an input within its normal range with all other available information. No manufacturer publishes P256B. Likely causes are a sticking or misadjusted switch, aftermarket idle controls wired into the same circuit, or a configuration change. The idle request is usually ignored.

Likely causes

Sticking or misadjusted idle selector
Aftermarket idle control wired into the same circuit
Idle or PTO configuration changed without matching hardware

Symptoms

  • Raised idle request ignored
  • Idle selection works only sometimes

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–300
Time0.5–2 h

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

Parts involved

idle speed selector switch

Related codes

P256AP256CP256DP256EP254B

References: law.cornell.edu, http:.