What it means
The ECU detects a low-side voltage below threshold on the heater control of the rear oxygen sensor on bank 1. The heater driver may be stuck on or shorted to ground.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
P2256 concerns the negative-current control circuit for the upstream oxygen sensor on bank 2, sensor 1. The module sets it when that circuit is below its lower threshold; the supplied meaning text describes a different rear-sensor heater fault, so confirm the code and vehicle-specific definition before testing.
Common causes
Most likely first.
- Heater wire shorted to ground
- Internal heater short
- Connector pins corroded - high contact resistance
Symptoms
- Check engine light on after several drive cycles
- Slightly increased fuel consumption during cold start
- Delayed entry into closed-loop fuel control
- Failed emissions inspection (lambda readiness)
How to diagnose it
- Capture freeze-frame data and live readings for bank 2 sensor 1, comparing them with bank 1 sensor 1 to establish when the circuit fault appears.
- Inspect the bank 2 sensor 1 connector and nearby harness for corrosion, poor pin contact, chafing, or heat damage.
- Use the wiring diagram to identify the Vs negative circuit and check for a short to ground or an open circuit.
- Disconnect the sensor as directed by service information and determine whether the low reading is on the harness side or sensor side.
- Test the sensor and circuit continuity against the manufacturer's specification before replacing parts.
Common fixes
- Clean or repair corroded terminals at the bank 2 sensor 1 connector.
- Repair a damaged or grounded Vs negative wire.
- Replace the bank 2 sensor 1 oxygen sensor if its internal negative-current circuit fails testing.
Can I keep driving?
The car is generally drivable, but fuel economy and emissions can worsen if the module cannot use the bank 2 upstream sensor correctly. Avoid extended driving if rough running develops and repair it before an emissions inspection.
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.