P0138 The downstream oxygen sensor on bank 1 is reporting a voltage that stays too high

O2 sensor circuit high voltage, bank 1 sensor 2

Monitor

The rear sensor mainly monitors the catalytic converter rather than controlling fuel, so the car normally drives the same; book a diagnosis rather than stopping.

It gets more serious if:

  • Stop: Stop if the check engine light starts flashing: that means an active misfire hot enough to damage the catalytic converter. Pull over where nothing flammable is under the car and let it cool.

Source: how we rate severity

P0138 in short

P0138 means the oxygen sensor after the catalytic converter on bank 1 is reading a voltage that stays too high. Common causes are a wiring short to voltage, a worn or contaminated sensor, or a genuinely rich engine. The car usually drives normally, but fix it before judging the converter.

The downstream sensor (bank 1 sensor 2) sits after the catalytic converter and checks that the converter is working. Its voltage normally rises when the exhaust is rich and falls when it is lean, and a healthy converter keeps it fairly steady.

P0138 sets when that voltage stays above the range a working sensor should produce. On the 2008 Saab 9-3, for example, it sets above about 1.08 volts for five seconds with a warm sensor. A signal wire shorted to voltage, a contaminated sensor or an engine that really is running rich can all do this.

Because the computer uses this sensor to grade the catalytic converter, a faulty reading can lead to a catalyst code and an unnecessary converter replacement. Diagnose other codes first, then the wiring, then exhaust leaks, and replace the sensor last.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Signal wire shorted to voltage (often to the heater wire)

    Common general cause
    Why it ranks here
    A signal wire touching a voltage source, such as the heater supply where insulation has melted, drives the reading above anything a sensor can produce. Saab's service information treats this code as an electrical fault.
    What would change this
    Damaged insulation, or a reading above about 1 volt with the sensor unplugged, confirms it.

  2. Failed or contaminated rear oxygen sensor

    Common general cause
    Why it ranks here
    A worn or contaminated sensor can stick at a high reading. Saab's procedure replaces the rear sensor once wiring, other codes and leaks are ruled out.
    What would change this
    A sensor that stays high even during deceleration, with good wiring, confirms it. Heavy deposits on the tip point to oil, coolant or a rich mixture.
    How to test the oxygen (o2) sensor

  3. Engine genuinely running rich

    Possible — needs testing
    Why it ranks here
    A rich engine makes the rear sensor read high. Saab's procedure has other codes diagnosed first for this reason.
    What would change this
    Rich codes (P0172) or strongly negative fuel trims raise this; normal trims lower it.

  4. Reference voltage fault at the sensor connector

    Possible — needs testing
    Why it ranks here
    Saab's procedure checks the reference voltage at the connector before condemning the sensor.
    What would change this
    Incorrect reference voltage with sound wiring raises this.

  5. Exhaust leak between the front and rear sensors

    Possible — needs testing
    Why it ranks here
    Saab's procedure checks the exhaust between the sensors before replacing the rear sensor; a leak upsets the reading the computer expects.
    What would change this
    A crack, hole or loose joint between the sensors raises this.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all codes; diagnose fuel-trim, misfire and other oxygen sensor codes first

    Low riskTool: OBD-II scan tool

  2. With the engine fully warm, read the rear (bank 1 sensor 2) voltage while letting the engine decelerate from a higher speed with the throttle closed; it should drop low as fuel is cut

    Use cautionTool: scan tool with live data

    Warning:Keep hands, hair and loose clothing clear of the belts and fan while the engine runs, and never run the engine in a closed garage.

    • IfThe voltage stays high during deceleration

      It meansThe signal is stuck high; check the wiring.

      Go to step 3

    • IfThe voltage drops normally now

      It meansThe fault is intermittent; check the exhaust and mixture.

      Go to step 4

  3. With the exhaust cold, inspect the rear sensor wiring and connector, then check reference voltage and signal-wire continuity

    Use cautionTool: digital multimeter; wiring diagram

    Warning:Let the exhaust cool fully before touching the sensor or its wiring. Back-probe connectors gently, never pierce insulation, and follow the wiring diagram for your engine.

  4. With the exhaust cold, check for cracks, holes or loose joints between the front and rear sensors

    Use cautionTool: visual inspection; smoke machine (shop)

    Warning:Exhaust parts stay hot enough to burn skin long after the engine stops. Let the car cool for at least an hour, and never get under a car held up only by a jack.

Symptoms

  • Check engine light with no change in how the car drives
  • Catalyst efficiency code (P0420) stored as well
  • Failed or refused emissions inspection

How to fix

  1. With the exhaust fully cold, inspect the rear oxygen sensor's wiring for melted insulation or chafing against the exhaust, and check its connector is latched and free of corrosion.

  2. Look along the exhaust between the front and rear sensors for holes, cracks or loose joints.

Parts and cost context

  • Oxygen (O2) sensor:Replace the rear sensor only after wiring, exhaust leaks and other codes are ruled out, and use the correct part for that position.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Chevrolet

  • 2018–2019 · Equinox

    Supported by vehicle data

    On 2018-2019 Equinox, GM has repair information for P0138 (with P0132) alongside start/stop and hesitation concerns.

  • 19NA077Status unverified

    P0132 and/or P0138 with start/stop and hesitation concerns

    2018–2019equinox · issued · NHTSA 10169267

    GM repair information for a lit MIL with P0132 and/or P0138 together with start/stop performance or hesitation from a stop.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0137The downstream oxygen sensor on bank 1 is reporting a voltage that stays too low

    Same system Common general cause

    P0137 is the opposite fault on the same rear sensor circuit and follows the same diagnostic path: other codes, wiring, exhaust leaks, then the sensor.

  • P0420Catalytic converter efficiency below the computer's limit (Bank 1)

    Primary / secondary Common general cause

    Likely primary: P0138. The catalyst test relies on this sensor. Fix P0138 first; Saab's procedure has other codes diagnosed before P0420, and a faulty rear sensor can fail a good converter.

  • P0172Engine computer has reached its limit removing fuel on bank 1: the mixture is rich

    Primary / secondary Possible — needs testing

    Likely primary: P0172. A genuinely rich engine pushes the rear sensor high. If P0172 is stored, fix the mixture first.

All Oxygen sensors codes

P0138 questions

Can I drive with P0138?

Usually yes; the rear sensor doesn't control fuelling on most cars, so the engine runs the same. Book a diagnosis, because the fault stops the converter from being checked and can lead to a failed inspection.

Which oxygen sensor is bank 1 sensor 1, and which is sensor 2?

Bank 1 is the side of the engine with cylinder 1; an inline engine has only bank 1. Sensor 1 is the upstream sensor before the catalytic converter; sensor 2 is the downstream sensor after it. Check a diagram for your engine before buying a part.

Will replacing the sensor fix P0138?

Only if the sensor is the fault. Wiring damage near the exhaust and exhaust leaks between the sensors set the same code, so check those first.

Can an oxygen sensor fault damage my catalytic converter?

It can, over time. The EPA notes that fixing a failing oxygen sensor promptly can save you from replacing the much more expensive catalytic converter later.

Will my car pass an emissions inspection with this code?

Not while the check engine light is on; a lit light with stored codes fails the federal OBD inspection. After the repair the light normally goes out after three good drives, and the oxygen sensor and catalyst monitors must complete before a retest.

Sources and review

  1. Bosch Oxygen Sensors consumer brochure — Robert Bosch LLC ,
  2. Saab 9-2X 2.5i (2005) WIS DTC P0138: O2 sensor circuit high voltage (bank 1 sensor 2) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  3. Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II) for 2004 and later passenger cars, light-duty trucks and medium-duty vehicles — California Air Resources Board
  4. 2026 Honda Pilot Owner's Manual: Indicator Coming On/Blinking (Malfunction Indicator Lamp) — American Honda Motor Co.
  5. Saab 9-3 (2008) WIS DTC P0138 00 (ME9): O2S 2 circuit high voltage — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  6. Bosch Oxygen Sensors trade brochure — Robert Bosch LLC ,
  7. P0171 Code: Causes, Symptoms, and How to Fix It — Identifix ,
  8. P0420 Code: The Guide to Diagnosis and Fast Fixes — Identifix ,
  9. Saab 9-2X 2.5i (2005) WIS DTC P0137: O2 sensor circuit low voltage (bank 1 sensor 2) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  10. Saab 9-2X 2.5i (2005) WIS DTC P0420: Catalyst system efficiency below threshold (bank 1) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  11. General Motors bulletin 19NA077: P0132 and/or P0138 with start/stop and hesitation concerns — General Motors (filed with NHTSA) ,
  12. A/F Sensors — DENSO Auto Parts
  13. 40 CFR §85.2222 Onboard diagnostic test procedures — U.S. Environmental Protection Agency (Electronic Code of Federal Regulations)
  14. OBD: Frequently Asked Questions (EPA420-F-02-014) — U.S. Environmental Protection Agency, Office of Transportation and Air Quality ,
Last reviewed
Written by
Saurabh K.
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