P256DEngine Idle Speed Selector Sensor/Switch Circuit High
- Generic OBD-II code · Powertrain
- How the car detects it
- 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.
- What you may notice
- Raised idle cannot be selected
- Normal driving unaffected
- Likely causes
- LikelySelector wire chafed against a battery-fed circuit
- PossibleOpen return line letting the input float high
- RareAccessory module feeding system voltage into the line
- Repair
- Easy · Often possible to do yourself.Typical cost: €20 – €250
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.