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P2255 – O2 Sensor Negative Current Control Circuit Low Bank 2 Sensor 1 (OBD2)

P2255 means "O2 Sensor Negative Current Control Circuit Low Bank 2 Sensor 1". The voltage the control unit measures in the oxygen (lambda) sensor circuit is below the permitted minimum. For a sensor this usually means the signal wire is shorted to ground or the reference voltage is missing; for an actuator, that its control wire is shorted to ground.

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P2255 quick summary

Fault codeP2255
MeaningO2 Sensor Negative Current Control Circuit Low Bank 2 Sensor 1
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityMedium
Can I drive?Yes, in most cases. Fuel control becomes less accurate, so fuel consumption and emissions rise and the car may fail an emissions test. Do not ignore it for months, as a wrong mixture can damage the catalytic converter.
Repair cost€40 – €350

How oxygen (lambda) sensors work

Oxygen sensors measure how much oxygen is left in the exhaust, which tells the engine control unit whether the mixture is rich or lean and how well the catalytic converter works.

Petrol engines have at least two oxygen sensors per exhaust bank: an upstream sensor (sensor 1) before the catalytic converter and a downstream sensor (sensor 2) after it. The upstream sensor is used for fuel control; the downstream sensor checks the catalytic converter. Diesels use wide-band oxygen sensors mainly for EGR and smoke control.

A conventional narrow-band sensor switches between about 0.1 V (lean) and 0.9 V (rich) several times a second when the engine is warm and running in closed loop. Wide-band (air-fuel ratio) sensors work differently and report a current or a lambda value. The sensor only works when hot, which is why it has a heater (see the separate heater codes).

Codes in this group cover electrical faults (low, high, no activity), slow response and swapped signals. A slow or inactive sensor makes fuel control less accurate, increases fuel consumption and emissions, and can lead to catalytic converter damage over time.

Bank 2 is the cylinder row that does NOT contain cylinder 1, so this code only appears on V6, V8, V10, V12 and boxer engines.

Sensor 1 is the upstream oxygen (lambda) sensor, mounted before the catalytic converter. It is the sensor the engine uses to control the mixture.

Symptoms of P2255

  • Check Engine light on
  • Higher fuel consumption
  • Rough idle or hesitation (upstream sensor)
  • Failed emissions test
  • Sometimes a smell of rotten eggs or catalytic converter codes such as P0420

Most common causes of P2255 (most likely first)

Mechanic's Pro Tip

An oxygen sensor rarely fails on its own – it usually reports a problem. Before replacing an upstream sensor that reads lean, check for vacuum and exhaust leaks; before replacing one that reads rich, check for leaking injectors or high fuel pressure. If the trims are normal and only the sensor signal is lazy, then the sensor itself is worn.

  1. 1.Signal or control wire shorted to ground

    Most likely

    Look for chafed insulation where the harness touches the engine, exhaust brackets or body edges.

  2. 2.Missing 5 V reference or supply voltage

    An open reference wire or a shorted sensor elsewhere on the same reference circuit can pull it down.

  3. 3.Internal short in the oxygen (lambda) sensor

  4. 4.Aged or contaminated oxygen sensor

    Sensors typically last about 100,000–160,000 km; oil, coolant or silicone contamination shortens their life.

  5. 5.Exhaust leak near the sensor

    A leak before the sensor sucks in outside air and makes the mixture look lean.

  6. 6.Wiring damage near the exhaust

    Heat and contact with the exhaust can melt or chafe the sensor wires.

  7. 7.Real mixture problem

    A vacuum leak, a leaking injector or low fuel pressure can make a healthy sensor report lean or rich.

Step-by-step diagnostic guide

  1. 1

    Read codes and fuel trims

    Read all codes and look at short- and long-term fuel trims. Large trims point to a real mixture problem rather than a faulty sensor.

  2. 2

    Measure the signal voltage

    With the ignition on and the oxygen (lambda) sensor plugged in, back-probe the signal wire. Then unplug it and check the reference/supply and ground wires separately.

    Multimeter: Signal near 0 V = short to ground or no reference. Reference wire: about 5 V with ignition on. Signal wire to ground with connector unplugged: no continuity.

  3. 3

    Watch the sensor signal

    With the engine warm at about 2,500 rpm, watch the sensor in live data. An upstream narrow-band sensor should switch rapidly; a downstream sensor should be fairly steady when the catalytic converter is working.

    Multimeter: Narrow-band upstream: switching between about 0.1 and 0.9 V several times per second. Downstream: steady, usually around 0.5–0.8 V.

  4. 4

    Check for exhaust leaks

    Check the manifold, the flexible joint and the gaskets before the sensor for leaks (soot marks, ticking noise).

  5. 5

    Check heater and wiring

    Check the heater circuit and the signal wiring, especially where the harness runs near the exhaust.

  6. 6

    Replace and verify

    Replace the sensor with the correct type (narrow-band or wide-band, correct cable length), use anti-seize on the thread only, clear codes and check the signal again.

Frequently asked questions

Can I keep driving with P2255?

Yes, in most cases. Fuel control becomes less accurate, so fuel consumption and emissions rise and the car may fail an emissions test. Do not ignore it for months, as a wrong mixture can damage the catalytic converter.

How much does it cost to fix P2255?

Typical repair costs are 40 € – 350 €, depending on the cause and the vehicle. The most common fixes are: signal or control wire shorted to ground; missing 5 V reference or supply voltage; internal short in the oxygen (lambda) sensor. Always confirm the cause with the tests above before buying parts.

What is the difference between sensor 1 and sensor 2?

Sensor 1 is upstream (before the catalytic converter) and controls the mixture. Sensor 2 is downstream (after the converter) and monitors converter efficiency. A sensor 2 fault does not usually affect how the engine runs.

Should I buy an original or universal oxygen sensor?

A direct-fit sensor from an OE supplier (such as Bosch, NTK/NGK or Denso) with the correct connector is the safest choice. Universal sensors require splicing wires, which is a common source of new faults.

What is the difference between P2255 and similar codes?

P2255 belongs to a group of codes for the same component that differ in the type or location of the fault: P2252 – O2 Sensor Negative Current Control Circuit Low Bank 1 Sensor 1; P2253 – O2 Sensor Negative Current Control Circuit High Bank 1 Sensor 1; P2254 – O2 Sensor Negative Current Control Circuit/Open Bank 2 Sensor 1; P2256 – O2 Sensor Negative Current Control Circuit High Bank 2 Sensor 1; P2270 – O2 Sensor Signal Biased/Stuck Lean Bank 1 Sensor 2; P2271 – O2 Sensor Signal Biased/Stuck Rich Bank 1 Sensor 2. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.