EN

Severity: Low Code type: Generic (all makes)

P2238 Code: O2 Sensor Positive Current Control Circuit Low B1S1

The pump-current wire of the upstream air-fuel sensor is being pulled low, usually by a short to ground.

At a glance

Severity: 2/5 – Low2/5

Severity: Low

Meaning
O2 Sensor Positive Current Control Circuit Low B1S1
System
Fuel & Air Metering · Powertrain
Safe to drive?
Yes, short term
Typical repair cost
$100–$470
DIY difficulty
Moderate

On engines with a wideband air-fuel sensor ahead of the catalytic converter, the ECM holds the pump-current wire (IP on Bosch-type sensors, AF+ on Denso-type sensors) at a set bias voltage and measures how much current flows to keep the sensor's reference chamber balanced. P2238 means the voltage on that wire dropped below what the ECM can hold, which almost always means it's touching ground or has been dragged down by another circuit. Typical culprits are a harness chafed through on an exhaust bracket, subframe or transmission, a sensor that has shorted internally (often after its heater failed or its element was contaminated), moisture bridging pins in the connector, or the pump wire shorted to the sensor's reference/common wire. Where P2237 is about a wire that's broken, P2238 is about one that's touching something it shouldn't, and the quickest way to tell sensor from harness is to unplug the sensor and see if the voltage recovers.

Can I keep driving? Safe to drive in the short term, since the ECM switches to open-loop fueling. Don't ignore it for months: prolonged rich running can overheat the catalytic converter, and a short that also blows a shared fuse can take other components with it.

How to read P2238

How to read P2238P12223384
  1. P Powertrain
  2. 2 Generic (SAE)
  3. 2 Fuel & air (injector circuit)
  4. 38 Specific fault

Symptoms

  • Check engine light, sometimes along with heater circuit codes
  • Scan tool shows the sensor stuck at an extremely rich value that fuel trims don't confirm
  • Fuel economy a few percent worse
  • Possible blown fuse for the sensor heater, which on some cars also feeds injectors or coils
  • O2 sensor and catalyst readiness monitors not completing

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Pump-current wire chafed through to ground on an exhaust bracket, subframe or transmission 30%
  2. Sensor shorted internally, often after heater failure or contamination 25%
  3. Moisture or corrosion bridging pins inside the sensor connector 18%
  4. Pump-current wire shorted to the reference/common wire (on Denso-type sensors, AF+ touching AF-) 12%
  5. Harness damaged or pinched during earlier exhaust or transmission work 10%
  6. ECM fault 5%

Diagnosis step by step

Diagnostic flow: P22381Read the live value and trimstogether2Measure the bias voltage3Unplug the sensor to split theproblem4Hunt the short in the harness5Check the heater fuse6Repair, replace and verify

  1. Read the live value and trims together

    With the engine warm, look at the air-fuel reading and fuel trims. A sensor pinned at full rich while the trims aren't pulling fuel out, and no black smoke or fuel smell, points to a circuit fault rather than a real rich mixture.

    Tools: OBD-II scanner

  2. Measure the bias voltage

    Key on, sensor connected, back-probe the pump-current and reference wires and compare with the specified bias voltages; on Toyota-style Denso sensors you'd expect roughly 3.3 V on AF+ and 3.0 V on AF-. A pump wire near 0 V confirms it's being pulled to ground.

    Tools: Multimeter

  3. Unplug the sensor to split the problem

    Disconnect the sensor and measure the pump-current pin on the harness side again. If the voltage returns to normal, the short is inside the sensor; if it stays near zero, the fault is in the harness or the ECM.

    Tools: Multimeter

  4. Hunt the short in the harness

    Key off, ECM and sensor unplugged, measure from the pump-current wire to chassis ground and to each of the other sensor wires; every reading should be open. Then inspect the harness where it crosses brackets, the transmission and the subframe for shiny rub-through spots.

    Tools: Multimeter, Flashlight

  5. Check the heater fuse

    Look at the fuse feeding the sensor heater. A blown fuse together with P2238 strongly suggests the sensor shorted internally between heater and element; replace the sensor before fitting a new fuse or it will blow again.

    Tools: Flashlight, Pliers

  6. Repair, replace and verify

    Repair the harness with sealed connections and re-secure it away from edges, or install the correct sensor type. Clear codes and confirm the reading moves around lambda 1.00 at idle and swings lean during a closed-throttle coast-down.

    Tools: O2 sensor socket, Torque wrench, OBD-II scanner

Where the parts are

Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2) – P22381212345
Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2)
  1. 1Upstream O2 sensor
  2. 2Catalytic converter
  3. 3Downstream O2 sensor
  4. 4Diesel particulate filter
  5. 5Turbocharger

Repair options & costs

RepairDIY?Estimated cost
Repair chafed harness and re-secure No $120–$350
Replace upstream air-fuel sensor, installed Yes $180–$450
Replace sensor connector or pigtail No $100–$250
Replace sensor and blown heater fuse Yes $190–$470

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Toyota

Toyota and Lexus engines with Denso air-fuel sensors use this code when AF+ is shorted to ground or to AF-. Their scan data shows sensor output as a voltage around 3.3 V at the ideal mixture, so a reading pinned far below that with normal trims is a strong wiring clue.

Emissions / smog test impact

Will fail

P2238 turns on the check engine light and keeps the O2 sensor monitor from completing, so the vehicle fails OBD-based state emissions tests until it's repaired and the monitors are ready again.

Frequently asked questions

What's the difference between P2237 and P2238?

P2237 means the pump-current wire is open, like a broken wire. P2238 means it's pulled low, usually because it's touching ground or another wire. The tests are different: continuity for P2237, shorts to ground for P2238.

Can a short in this circuit damage the ECM?

The ECM output is designed to tolerate a short to ground, and it sets the code to protect itself. Still, fix the short promptly, and never try to 'test' the circuit by jumping it to battery voltage.

Why did this start right after exhaust work?

Exhaust and transmission jobs often mean unclipping the sensor harness. If it's put back too close to a sharp bracket or a hot pipe, it can wear through or melt within weeks.

Will the catalytic converter suffer?

Not immediately, but the ECM runs slightly rich without feedback. Over months that adds heat and deposits in the converter, so it's worth fixing soon.

Sources