Fuel & air metering · Powertrain · generic (SAE-defined)
P0151: O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)
P0151 means the upstream oxygen sensor on bank 2 is reporting persistently low voltage, which looks like a constant lean mixture or a shorted signal circuit.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0151 sets
The powertrain control module (PCM) monitors the bank 2 sensor 1 oxygen (O2) sensor, which sits ahead of the catalytic converter on the side of the engine opposite cylinder 1. A conventional sensor reads low when the exhaust is lean. If the signal stays below its expected range for a calibrated time during warm, closed-loop operation, P0151 sets. Limits are manufacturer-specific.
Common causes of P0151
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Bank 2 sensor 1 signal wire shorted to ground
- Failed bank 2 sensor 1 oxygen sensor
- Exhaust leak at the bank 2 manifold or gasket
- Real lean condition on bank 2, such as an intake gasket leak
- Sensor contamination from coolant or silicone
- Connector corrosion or poor terminal contact
P0151 symptoms
- Check engine light on
- Possible hesitation or surging
- Reduced fuel economy
- P0174 may be stored alongside it
How P0151 is diagnosed
- Scan for codes; if P0174 is also present, investigate a true lean condition on bank 2.
- Inspect the bank 2 upstream sensor harness for chafing and contact with hot exhaust.
- Check the bank 2 exhaust manifold for cracks or leaking gaskets.
- On live data, enrich the mixture briefly; a working sensor should swing to a rich reading.
- Compare bank 1 and bank 2 fuel trims to see whether the lean reading is isolated to bank 2.
- Test the signal circuit for a short to ground with the sensor unplugged.
Typical fixes for P0151
- Repair shorted or damaged wiring
- Replace the bank 2 sensor 1 oxygen sensor
- Repair bank 2 exhaust leaks
- Fix vacuum or intake leaks affecting bank 2
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
P0151 FAQ
What does code P0151 mean?
P0151 means the upstream oxygen sensor on bank 2 is reporting persistently low voltage, which looks like a constant lean mixture or a shorted signal circuit. The standard definition is "O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)."
Can I drive with a P0151 code?
Typically yes for short distances. Because the engine computer may add fuel to bank 2 based on a false lean reading, have it checked within a few days.
How do I tell if P0151 is the sensor or a real lean condition?
Look at bank 2 fuel trims. If trims are strongly positive and the sensor still responds when fuel is added, the mixture is likely truly lean; if the sensor never responds, the sensor or its wiring is the more likely fault.
Which side is bank 2?
Bank 2 is the cylinder bank that does not contain cylinder 1. Cylinder numbering differs by manufacturer, so confirm it in service information for your engine.
Keep researching
Engine guides by brand (known issues, specs)
More fuel & air metering codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.