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P0151 Generic (SAE)

O2 Sensor Circuit Low Voltage (Bank 2, Sensor 1)

Updated 2026-10-02 · Fuel & air metering

Severity
Medium
Typical repair cost
40 – 400 EUR
System
Powertrain

What it means

The upstream oxygen sensor on bank 2 (the bank without cylinder 1) is reporting a voltage stuck below about 0.1-0.2 V for longer than the ECU allows. A low voltage means the sensor sees excess oxygen – either because that bank genuinely runs lean, or because the signal wire is shorted to ground. This is a signal-circuit code; the heater has its own monitor (P0155).

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Symptoms

  • Check engine light, sometimes with P0171/P0174 lean codes
  • Hesitation or stumbling on acceleration from bank 2 running lean
  • Rough idle and occasional misfire counts on bank 2 cylinders
  • Higher than normal fuel consumption once the ECU goes to default fueling
  • Popping or backfire from the exhaust on deceleration

Common causes

  1. Exhaust leak before the sensor (cracked manifold, failed gasket) feeding oxygen past the sensor
  2. Genuine lean condition on bank 2: vacuum leak at the intake manifold gasket or a leaking injector seal on that bank
  3. Signal wire shorted to ground or insulation rubbed through on the exhaust
  4. Worn sensor with low output after 150,000+ km
  5. Low fuel pressure or a clogged injector affecting that bank
  6. Faulty sensor ground or ECU pin

How to diagnose

  1. Compare B1S1 and B2S1 live on the scan tool at 2000 rpm: if bank 1 switches 0.1-0.9 V normally while bank 2 is pinned below 0.2 V, concentrate on bank 2 only
  2. Check long-term fuel trim for bank 2: a value above +10% confirms a real lean condition, whereas near 0% with a low sensor voltage points to a wiring or sensor fault
  3. Spray propane or brake cleaner around the bank 2 intake runners and injector seals at idle – if the sensor voltage jumps up, you have found a vacuum leak
  4. Disconnect the sensor and read the ECU bias voltage at the harness; it should be about 0.45 V – if it reads 0 V the signal wire is grounded somewhere
  5. Snap the throttle: a good sensor should spike above 0.8 V within a second; a sluggish or flat response confirms a worn element
  6. Check fuel pressure at the rail (typically 3.5-4.0 bar on a returnless petrol system) and run an injector balance test to rule out a lean cylinder

Frequently asked questions

Is P0151 always a bad sensor?

No. A low voltage is often the sensor correctly reporting a lean condition caused by an exhaust leak or vacuum leak on bank 2. Check fuel trims before buying parts.

Why only bank 2 and not bank 1?

Each bank has its own exhaust manifold, intake runners and sensor, so a cracked manifold or leaking gasket on one side affects only that bank's sensor.

How much is a replacement?

A quality upstream sensor costs 60-180 EUR; labour is 30-60 minutes unless the sensor is seized in the manifold.

Related codes

P0150P0152P0154P0174P0131

Nearby codes in this group