What it means
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.
At a glance
- Usually lights the check-engine light
How it works
A/C request input B carries a request from the HVAC controls to the vehicle control module. The module checks whether this electrically present input agrees with the other system conditions; P256F sets when the B-channel value is implausible for those conditions.
Common causes
Most likely first.
- HVAC module sending inconsistent request B
- Sensor element aged beyond calibration window
- Sensor mounting loose - signal damping
Symptoms
- Check-engine light, if the fault triggers it
- Air conditioning may not engage
- A/C operation may be intermittent
How to diagnose it
- Check freeze-frame data and A/C request live data for channel B, comparing it with channel A and the HVAC command when the fault occurred.
- Confirm the A/C controls and HVAC module are actually requesting cooling, since a missing request can make the B input appear inconsistent.
- Inspect the B-channel connector, wiring, and sensor mounting for looseness, corrosion, damage, or movement that could damp or alter its signal.
- Use a scan tool to see whether the B input changes smoothly as the HVAC request changes and whether it tracks the expected system state.
- If the signal remains implausible, test the B-channel wiring and sensor against the manufacturer's specifications before considering an HVAC module fault.
Common fixes
- Secure the loose B-channel sensor or connector.
- Clean or repair corroded or damaged B-channel wiring and terminals.
- Replace the B-channel sensor if its output is outside the manufacturer's specification.
- Repair or replace the HVAC module if its B request output is inconsistent after circuit checks.
Can I keep driving?
If the A/C is the only affected system, the vehicle is generally drivable, but cooling may be unavailable or intermittent. Avoid relying on the A/C for defogging if it is not working.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.