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P2237 to P2240: wideband oxygen sensor pump current circuit
Wideband sensors work differently from the switching sensors most guides describe, and testing them the old way gives an answer that looks alarming and means nothing.
Most explanations of oxygen sensors describe a narrowband device: a voltage that swings between roughly 0.1 V and 0.9 V as the mixture crosses stoichiometric. Fast, cheap, and only accurate very close to that one point.
Modern engines — particularly direct injection petrol and all diesels — use wideband sensors, which measure the actual air-fuel ratio across a wide range. They work on a completely different principle.
| Code | Meaning |
|---|---|
| P2237 | O2 sensor pump current circuit / open, bank 1 sensor 1 |
| P2238 | Pump current circuit low |
| P2239 | Pump current circuit high |
| P2240 | Bank 2 sensor 1 equivalent |
Related: oxygen sensor lambda OBD diagnostics.
How a wideband sensor actually works
Two cells rather than one.
A measuring cell compares exhaust gas with a reference chamber. A pump cell electrically pumps oxygen into or out of that chamber to hold the measuring cell at a constant target.
The current required to do that pumping is the measurement. More current means more oxygen had to be moved, which tells you exactly how lean the mixture is — quantitatively, not just “lean”.
That pump current is what P2237 to P2240 refer to.
Why the old test does not apply
If you probe a wideband sensor expecting the 0.1–0.9 V switching of a narrowband, you will see something that looks wrong regardless of the sensor’s condition.
Wideband sensors typically have five or six wires: heater supply and ground, pump cell, reference cell, and a calibration resistor.
That calibration resistor matters. It is specific to the individual sensor and tells the module its exact characteristics. It is one reason universal wideband sensors and spliced repairs cause trouble that narrowband ones do not.
Read lambda, not voltage
The useful value in live data is lambda, or equivalence ratio.
| Lambda | Meaning |
|---|---|
| 1.00 | Stoichiometric |
| Above 1.00 | Lean |
| Below 1.00 | Rich |
| 1.4–1.6 at diesel idle | Normal — diesels always run lean |
| 0.85–0.90 at full throttle petrol | Normal enrichment for cooling |
A diesel showing lambda well above 1 is not faulty. Diesels run lean by design, and reading that number as a fault is one of the more common misinterpretations.
Causes
The sensor
Wideband sensors are more complex and more expensive than narrowband, and they still live in the exhaust. Failure with age is expected.
Contamination
Same poisons as any oxygen sensor: coolant from a head gasket, oil from worn seals, silicone from the wrong sealant. Wideband sensors are, if anything, more sensitive to contamination because the measurement is quantitative.
The connector
Five or six pins in a connector under the car. Corrosion adds resistance, and added resistance in a current-measuring circuit produces a wrong measurement rather than an obvious failure.
Wiring repairs done badly
A spliced wideband sensor frequently does not work correctly, even when every connection is sound, because the calibration resistor no longer matches. Use the correct sensor with its own connector.
An exhaust leak
Air entering upstream of the sensor makes it read lean, accurately, for the gas it is sampling.
Symptoms
- Poor fuel consumption.
- Hesitation or rough running.
- Mixture codes alongside: P0171 and P0174 lean mixture.
- On a diesel, incorrect EGR rate and increased smoke.
- Emissions monitors not completing.
Diagnostic sequence
- Read lambda in live data, not raw voltage.
- Cross-check against fuel trims. A lean reading with negative trims is a contradiction, and the sensor is the suspect: P2195 and P2197 oxygen sensor biased lean.
- Check heater operation — cold sensors read wrong: P0030 to P0037 heater circuit.
- Inspect the connector thoroughly.
- Check for exhaust leaks upstream.
- Replace with the correct part, never a universal with cut wires.
- Clear trims and confirm lambda settles where it should.
This check needs no workshop — an ELM327 adapter and the free tier of the app cover it. Read lambda at idle, at cruise and at full throttle. Those three numbers tell you more about a wideband sensor than any resistance measurement.
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