P256F
A/C Request B Circuit Range/Performance
A/C request input B is electrically intact but its value does not match the system context — same failure mode as the input A performance fault, applied to the second HVAC channel.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- HVAC module sending inconsistent request BCommon
- Sensor element aged beyond calibration windowLess likely
- Sensor mounting loose - signal dampingLess likely
Parts involved
- AC Request Input B
P256F is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.