P2256
O2 Sensor Heater Control Circuit Low Bank 1 Sensor 2
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.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
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)
Common causes
Ordered by how often each turns out to be the cause.
- Heater wire shorted to groundCommon
- Internal heater shortPossible
- Connector pins corroded - high contact resistanceLess likely
Parts involved
- Oxygen Sensor
- Oxygen Sensor Heater Circuit
P2256 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.