P256D code meaning
Engine Idle Speed Selector Sensor/Switch Circuit High
What P256D means
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.
P256D at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P256D
- Raised idle cannot be selected
- Normal driving unaffected
Common causes of P256D
- Most likelySelector wire chafed against a battery-fed circuit
- PossibleOpen return line letting the input float high
- Less likelyAccessory module feeding system voltage into the line
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Idle speed selector switch
What each part of P256D means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 5 | Area: vehicle speed, idle control and auxiliary inputs. |
| 6D | The exact fault in that area. |
What to do next
- Note P256D and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P256D
What does P256D mean?
P256D means "Engine Idle Speed Selector Sensor/Switch Circuit High". P256D is logged when the idle speed selector signal rises above the highest value the controller treats as valid. It is a generic code, so it means the same on every make.
Is it safe to drive with P256D?
P256D does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P256D fail an emissions test?
P256D is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P256D to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P256D clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.