Parts
$40–120
P0151 — O2 Sensor Low Voltage — Bank 2, Sensor 1
CAUTIONIs it safe to drive? The ECU is losing upstream O2 feedback on the second bank — fuel trims go open-loop and fuel economy and emissions suffer; fix within a week or two.
What P0151 means
The upstream (pre-catalyst) O2 sensor on Bank 2 is reading below 0.1V continuously or switching only in the lean range. Bank 2 is the side of the engine that does NOT contain cylinder number 1.
Most likely causes (in order)
- Failed upstream O2 sensor ($40–120 for most V6/V8 applications)
- Exhaust leak at or upstream of the sensor bung
- Vacuum leak on the Bank 2 side (lean exhaust reads as low voltage)
- Lean fuel injector fault on Bank 2 — check for P0204 companion codes
- Open signal wire to the ECU
Symptoms you might notice
- Check engine light on
- Possible lean hesitation under load
- Rough idle if the lean condition is significant
- Worse fuel economy
What to check first
Check for exhaust leaks near the sensor first — a leak pulling fresh air into the exhaust stream will fake a lean/low-voltage reading. If no leak, watch live Bank 2 fuel trims; positive long-term fuel trim above +10% confirms a real lean condition. Address the lean condition before condemning the sensor.
Repair cost & difficulty
Labor
0.5–1.0 hr
Difficulty
DIY-friendly