OBD-II trouble code
P2238: O2 Sensor Positive Current Control Circuit Low (Bank 1 Sensor 1)
Pump current below the expected window on the upstream sensor. The cause that catches people out is not a wire at all: many wideband sensors carry an individual calibration resistor inside the connector, so the connector is part of the sensor and cannot be swapped.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $110 – $750
- DIY difficulty
- Advanced DIY
What does P2238 mean?
The upstream sensor on a modern petrol engine measures mixture by pumping oxygen into or out of a tiny internal chamber and reporting how much current that took. This code says the module measured current on the positive side of that pump circuit below what it expected for the conditions it was commanding. It is a plausibility judgement, not a simple open or short — the module knows what current the loop should have needed and the number that came back was too small.
Because the module drives this circuit in both directions depending on whether the exhaust is lean or rich, the word "low" needs context to be useful. A low value while the module was commanding a large correction is a different story from a low value when it was barely working at all. Freeze frame data is worth more here than on most codes for that reason: engine load, coolant temperature, and the commanded mixture at the moment the fault set will tell you whether the loop was under real demand or idling along.
The cause that surprises people has nothing to do with the wiring. Many wideband sensors are individually calibrated at manufacture, and the calibration value is not stored in the module — it is a laser-trimmed resistor built into the sensor's own connector body. The module reads that resistor and uses it to interpret the pump current from that specific sensor. The consequence is that the connector is not a connector, it is part of the sensor, and three common practices break the relationship:
Fitting a used sensor pulled from another vehicle brings a calibration that belongs to a different sensor's element. Splicing a new sensor onto an old pigtail — which people do when the original connector is damaged and a replacement seems easier — hands the module the wrong trim value entirely. And swapping connector bodies between two sensors of the same part number, which looks harmless because the parts are physically identical, mismatches both of them. In every one of those cases the wiring is intact, the sensor works, and the current figures still land outside the window the module expects. Nothing you can measure at the harness will explain it. If a sensor has been replaced recently and this code appeared afterwards, the calibration relationship is the first thing to question, not the last.
When the fault is genuinely electrical, the character to look for is a leakage path rather than a dead short: something drawing a little current away from the circuit, or a partial path to ground that does not remove the signal so much as bleed it. Moisture and salt inside a connector do exactly that, which is why this code has a habit of appearing after wet weather or pressure washing and then disappearing for a fortnight. An intermittent that tracks the weather is a strong argument for opening the connector and drying and inspecting it rather than replacing hardware. Check that the connector seals and the wire seals are seated and undamaged, because on an upstream sensor the connector often sits low and forward where it collects spray.
Driving with it is acceptable in the short term. The module will either use a degraded interpretation of the sensor or ignore it and fuel the engine from programmed values, so the vehicle remains usable, but economy falls off and emissions monitors will not complete.
Common causes
- A used or salvaged sensor fitted, bringing calibration data that belongs to a different element
- New sensor spliced onto the old pigtail, so the module reads the wrong calibration resistor
- Connector bodies swapped between two physically identical sensors, mismatching both calibrations
- Moisture and salt inside the connector creating a leakage path away from the pump circuit
- Partial short to ground along the harness through damaged insulation
- Corroded or damaged terminals in the sensor connector
- Degraded pump cell inside a high-mileage sensor drawing less current than specification
- Damaged or unseated connector seals allowing repeated water entry
Symptoms
- Check engine light, often appearing shortly after an oxygen sensor was replaced
- The fault coming and going with wet weather, car washing or pressure washing
- Fuel economy down and mixture control less stable than normal
- A mixture value on the scan tool that is offset or implausible rather than absent
- Bank 1 fuel trim drifting even though there is no vacuum or fuel supply fault to find
- Emissions readiness monitors that will not complete
- Rough idle or light hesitation in some applications
- No misfire and no serious loss of power
Diagnostic steps
- 1.Read freeze frame first and note load, coolant temperature and the commanded mixture when the fault set. A low current under heavy demand means something different from a low current at light load.
- 2.Establish the sensor's history. If it was replaced recently, find out whether the part was new, whether the original connector was reused, and whether anything was spliced.
- 3.Inspect the sensor connector body for damage and confirm it is the one supplied with that sensor. On many wideband sensors the calibration resistor lives inside it and cannot be interchanged.
- 4.Open the connector and look for water, salt and green corrosion. A leakage path here explains both the low reading and any weather-related intermittency.
- 5.Check that the connector seals and individual wire seals are present, seated and undamaged.
- 6.Test the harness for a partial path to ground along the pump circuit rather than only checking for a dead short.
- 7.Measure terminal condition and retention. Contacts that have lost tension behave badly in a milliamp circuit.
- 8.Follow the manufacturer's pump circuit test procedure at the module connector. Do not apply voltage to the pump circuit outside that procedure, since it can destroy the sensor.
- 9.After repair, clear the code, bring the engine to full operating temperature and confirm the mixture value tracks sensibly through a range of loads before calling it fixed.
Repair cost
$110 – $750
Diagnosis is $100 to $250. If the cause turns out to be a calibration mismatch from a used sensor or a spliced pigtail, the correct fix is a new complete sensor with its own connector at $80 to $350 in parts and $160 to $600 fitted — and no amount of wiring work will substitute for it. Cleaning and resealing a wet connector is $60 to $200 and is a genuine repair when the terminals are still sound. A harness repair for a leakage path runs $130 to $400. Replacing the sensor before checking the connector for water is the most common wasted spend on this code, because a clean, dry, properly sealed connector resolves a meaningful share of them.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.