P0151
OBD-II Diagnostic Code
Severity: ModerateWhat it means
P0151 means the upstream oxygen sensor on Bank 2 is reading abnormally low voltage.
The sensor may be stuck lean or have failed.
Common causes are a vacuum leak, exhaust leak near the sensor, or a failed sensor.
Affected Models
- All OBD-II vehicles with V6/V8 engines (1996+)
Common Causes
- O2 sensor failed or aged
- Exhaust leak before the sensor
- Vacuum leak causing lean condition
- Fuel injector problem on Bank 2
- Sensor wiring issue
How to Fix It
-
Check for exhaust leaks near the Bank 2 upstream sensor.
An exhaust leak introduces outside air, making the sensor read lean (low voltage).
Listen for ticking or hissing from the exhaust manifold. -
Check for vacuum leaks.
A vacuum leak causes a lean condition that affects the O2 sensor reading.
Spray carburetor cleaner around vacuum hoses — a rise in RPM indicates a leak. -
Replace the O2 sensor if it has over 100,000 miles.
O2 sensors degrade over time.
A sluggish or stuck sensor should be replaced.
Bank 2 Sensor 1 is the upstream sensor on the side opposite cylinder 1.
Frequently Asked Questions
What voltage should the O2 sensor read?
A healthy O2 sensor oscillates between 0.1V (lean) and 0.9V (rich).
P0151 means it is stuck near 0.1V — always reading lean.
Which side is Bank 2?
Bank 2 is the side of the engine that does NOT contain cylinder 1.
Check your service manual for your specific engine.