P0151Powertrain
Powertrain · SAE
O2 Sensor Bank 2 Sensor 1 – Low Voltage
Meaning
The upstream lambda sensor on Bank 2 reports voltage consistently below the normal operating range (below ~0.1 V), indicating a permanent lean signal. The ECU interprets this as a sensor stuck at minimum voltage or chronically lean exhaust gas.
Common causes
- Persistently lean mixture on Bank 2 (vacuum leak, injector fault, insufficient fuel pressure)
- Open circuit in the signal wire holding voltage at 0 V
- Exhaust leak upstream of the sensor diluting exhaust gas with ambient air
- Signal wire short to ground
- Failed O2 sensor with signal stuck low
Symptoms
- MIL (Check Engine) warning light on
- Long-term fuel trim (LTFT) on Bank 2 strongly positive (fuel addition)
- Possible increased fuel consumption and power reduction
- Potentially elevated NOx emissions
- Possible rough engine behaviour
Severity
Moderata: The engine operates with compensatory rich corrections; if the signal reflects a genuinely lean mixture, long-term engine damage is possible.
What to do
- Read DTCs and fuel trims (STFT/LTFT) for Bank 2; a strongly positive LTFT suggests a real lean condition, not just a sensor fault.
- Check for exhaust leaks upstream of sensor B2S1 (noise or abnormal smell).
- Measure sensor voltage at idle: must oscillate between ~0.1 V and ~0.9 V; a fixed low value indicates a sensor fault.
- Check signal wire continuity and confirm no shorts to ground.
- Correct any exhaust leaks or fuel supply issues before replacing the sensor; replace sensor B2S1 if the electrical circuit is intact.