P0139 Code: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)

P0139 Code: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)
P0139 O2 Sensor Slow Response: Diagnosis and Fix
TEST FIRST — P0139 DOES NOT AUTOMATICALLY CONDEMN THE SENSOR

P0139 Code: The Bank 1 Downstream O2 Sensor Responds Slowly

P0139 means the PCM saw Bank 1 Sensor 2 take too long to react during a defined exhaust transition. Prove leaks, wiring, heater operation and the VIN-specific monitor before replacing the sensor or catalyst.

Updated August 18, 2026About 10 min readReviewed by iCARZONE Tech Team
QUICK ANSWER

P0139 means the PCM judged the Bank 1 Sensor 2 oxygen-sensor circuit response too slow during a defined monitor. This is the downstream sensor after the Bank 1 catalyst—not the upstream fuel-control sensor. Save the event, verify the exhaust transition and test leaks, wiring, heater operation, sensor response and applicable calibration before replacing parts.

What P0139 Means — A Slow Downstream Response

The standardized description is O2 Sensor Circuit Slow Response (Bank 1, Sensor 2). Bank 1 is the side containing cylinder 1; an inline engine generally has only Bank 1. Sensor 2 is the first oxygen sensor after that bank’s catalyst. Multi-catalyst or three-sensor layouts must be confirmed from the VIN-specific diagram.

The PCM sets P0139 when B1S2 takes longer than expected to follow an exhaust-oxygen change under calibrated conditions. It is not a direct circuit-open, low-voltage, high-voltage, no-activity or heater verdict, and it does not prove the sensor or catalyst is bad.

1. Defined Event

The PCM observes a calibrated exhaust transition, often under conditions such as deceleration fuel cut.

2. Delayed Response

B1S2 does not begin or complete the expected response within that calibration.

3. System Context

Leaks, heater status, catalyst behavior, mixture and wiring decide what the slow graph means.

Do not compare B1S2 with a universal upstream switching rule. A functioning catalyst normally smooths downstream oxygen changes. Sensor technology, PID scaling, response direction and limits vary, so a generic 0.1–0.9 V rapid-switching test can misdiagnose the system.
P0139: B1S2 Slow ResponseP013A/P013B: Direction-Specific B1S2P0140: B1S2 No ActivityP0159: B2S2 Slow Response

P0139 Symptoms, Severity, and Driving Safety

A steady MIL and emissions-readiness concern are the most defensible symptoms. Driveability complaints usually come from a companion mixture, misfire, exhaust or catalyst problem rather than the downstream slow-response code alone.

Check Engine Light
A steady MIL may be the only symptom. P0139 often appears without a noticeable change in how the vehicle starts, idles or accelerates.
No Obvious Driveability Change
Bank 1 Sensor 2 usually supports catalyst and emissions monitoring rather than primary fuel control, so normal driving can continue despite the stored code.
Emissions or Readiness Failure
An active emissions DTC can prevent an inspection pass. Clearing the code before diagnosis also resets readiness and removes useful monitor evidence.
Companion O2 or Catalyst Codes
P0136–P0141, P0133, P0420, fuel-trim or misfire codes can show whether the slow response is part of a broader electrical, exhaust or combustion problem.
Fuel Economy or Performance Change
Poor economy, hesitation or roughness points toward a real mixture, misfire or exhaust problem rather than P0139 alone. Diagnose the complete code set.
Fuel, Sulfur, or Exhaust Odor
A strong odor is not a normal P0139 requirement. It can indicate rich operation, a hot catalyst, an exhaust leak or another fault needing faster attention.
Stop or sharply limit driving for a flashing MIL, severe misfire, major power loss, a glowing or overheated catalyst, smoke, a strong raw-fuel odor or exhaust fumes entering the cabin. A stable vehicle with only a steady MIL can normally be driven cautiously for prompt diagnosis.

Six P0139 Causes in Test Order

This order proves that the exhaust event reached a correctly heated, electrically sound B1S2 before the sensor or catalyst is condemned. It is a diagnostic sequence, not a claimed failure-rate ranking.

1

Exhaust Leak Before or Around B1S2

Fresh air entering through a flange, weld, flex joint, catalyst seam or sensor seat can distort the oxygen transition that the PCM expects to observe.

How to prove it: With the system cool, inspect the full path ahead of B1S2 and use an OEM-approved low-pressure exhaust-leak method. Repair a proven leak before judging sensor speed.

Check first
2

Heat-Damaged Harness or Poor Connector

Melted insulation, tension, chafing, water, corrosion or a weak terminal can slow, bias or intermittently interrupt the downstream sensor signal.

How to prove it: Inspect hot-zone routing, connector seals and terminal fit. Then prove the signal and ground paths by the VIN-specific diagram and approved probing method.

Check first
3

Heater Supply, Ground, or Control Fault

A downstream sensor that has not reached its intended temperature may respond slowly. P0141 or a shared-power code increases this branch priority.

How to prove it: Check the heater circuit under the exact OEM conditions. Do not apply a universal resistance or current value to every sensor design.

Test directly
4

Aged, Contaminated, or Incorrect B1S2

Oil, coolant, silicone, sustained rich operation, physical shock or the wrong replacement specification can reduce downstream sensor response.

How to prove it: First prove the exhaust transition, circuit and heater. Replace B1S2 only when its response or electrical test fails while the system input is known good.

Test directly
5

Mixture, Misfire, Catalyst, or Upstream-Sensor Problem

A transition that never reaches the catalyst as expected—or abnormal catalyst oxygen storage—can make the downstream monitor fail even when B1S2 can respond.

How to prove it: Use companion P017x, P030x, P013x and P0420 evidence plus fuel trim and upstream/downstream data. Do not condemn the catalyst from P0139 alone.

Test directly
6

VIN-Specific Calibration or Rare PCM Fault

Some OEM bulletins correct an overly sensitive P0139 monitor on narrowly defined vehicles. Controller hardware is the final branch after external tests pass.

How to prove it: Check VIN, engine, calibration ID and bulletin applicability. Apply software or controller work only when the current OEM procedure directs it.

Final branch

Software is VIN-specific. Honda, Mitsubishi and Infiniti have documented narrow P0139 calibration branches on selected vehicles. Their existence proves that monitor logic can matter; it does not make programming the universal first repair.

Using UR1000 for P0139 Scan Evidence

The iCARZONE UR1000 can save supported engine DTCs, freeze frame, readiness, available O2 monitor information, live data and diagnostic reports. On compatible vehicles, graph B1S1 and B1S2 beside fuel trim, RPM, load and relevant monitor status to show whether the downstream sensor responded when the test event occurred.

Coverage and PID names vary by make, model, year, VIN, ECU and software. UR1000 cannot physically detect an exhaust leak, inspect a connector or measure voltage, resistance, current, voltage drop or waveform. Those checks require the correct diagram, exhaust-leak equipment, meter and scope.

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How to Diagnose P0139 in 6 Steps

Allow the exhaust and catalyst to cool before inspection. Support a raised vehicle correctly and use approved exhaust extraction for running tests. A helper should handle live road data, or record it for review after the drive.

Compact P0139 Test Path

  • 1

    Save the Complete Event

    • Record all-module DTCs, stored/pending/permanent status, freeze frame, readiness and any available oxygen-sensor monitor result before clearing anything.
    • Note mileage, recent exhaust or sensor work, oil or coolant use, refueling, battery events and the exact operating conditions when the MIL appeared.
  • 2

    Confirm Bank 1 Sensor 2 and the Monitor

    • Use VIN-specific cylinder numbering and exhaust diagrams to locate Bank 1 and the first downstream sensor after its catalyst. Do not guess left or right.
    • Identify sensor type, supported PID, monitor direction and enable conditions. P0139 is a general slow-response code; P013A/P013B are direction-specific B1S2 codes.
  • 3

    Resolve Companion Codes and Inspect Cold

    • Prioritize system-voltage, heater/circuit, misfire, fuel-trim, upstream O2 and catalyst-related codes as the OEM chart directs.
    • After the exhaust cools, inspect the sensor, connector, harness routing, exhaust joints and contamination. Confirm the installed part and connector are correct.
  • 4

    Prove the Exhaust Path Is Sealed

    • Check the sensor seat, flanges, flex section, welds and catalyst seams ahead of B1S2 with an approved low-pressure exhaust-leak method.
    • Do not use a generic intake smoke test as proof of exhaust sealing, and never introduce flammable chemicals into a hot or running exhaust.
  • 5

    Prove Heater, Circuit, and Sensor Response

    • Use the OEM diagram, meter or scope to test heater power/control, ground, signal integrity, terminal tension and loaded voltage drop. Never apply battery voltage to the signal circuit.
    • On supported vehicles, graph B1S1, B1S2, fuel trim, RPM and relevant status data with UR1000 during the specified monitor event. A downstream sensor should not be judged by a universal rapid-switching rule.
  • 6

    Repair, Clear, and Complete the Monitor

    • Repair only the proven exhaust, wiring, heater, sensor, mixture, catalyst or calibration branch. Use correct sensor handling, anti-seize guidance and torque, plus stable support voltage if programming is required.
    • After saving evidence, clear eligible codes and complete the VIN-specific drive cycle. Confirm readiness returns and no current or pending P0139 or companion code reappears.
Do not apply battery power to an oxygen-sensor signal circuit or force mixture changes with flammable chemicals. Do not widen terminals with oversized probes, and do not use an O2 spacer or software deletion to conceal an emissions fault.

How to Fix, Clear, and Budget for P0139

01

Seal the Proven Exhaust Leak

Repair the verified sensor-seat, flange, flex, weld or catalyst-seam leak ahead of B1S2, then retest response.

02

Restore Wiring and Heater Operation

Repair damaged terminals, heat routing, signal or ground conductors and the proven heater supply or control fault.

03

Replace Only a Proven Slow B1S2

Use the correct sensor specification only after the exhaust input, heater and electrical paths are known good.

04

Repair the System or Calibration Cause

Correct companion mixture, misfire or catalyst faults, or apply an applicable OEM software branch by VIN.

After repair, complete any required sensor or controller setup, clear eligible codes and run the exact OEM monitor or drive cycle. Confirm current and pending P0139 stay absent and oxygen-sensor readiness returns. A permanent DTC is not manually erased; it clears only after the vehicle satisfies its own successful OBD criteria.

01

Scan and Response Diagnosis

VIN-specific quote
Main cost driver

Monitor reproduction, data capture and separating exhaust input from sensor delay.

02

Exhaust Leak Repair

VIN-specific quote
Main cost driver

Leak location, corrosion, catalyst or flex-section access and fabrication needs.

03

Connector, Harness, or B1S2

VIN-specific quote
Main cost driver

Sensor technology, seized threads, hot-zone wiring and terminal serviceability.

04

System or Calibration Repair

VIN-specific quote
Main cost driver

Companion-code diagnosis, catalyst evidence, bulletin applicability and programming.

P0139 does not identify a universal failed part or price. Request a written estimate based on the VIN, proven cause, exhaust access, sensor type, corrosion, local labor and emissions coverage.

Vehicle Differences and DIY Limits

Sensor 2 can sit directly behind a close-coupled catalyst or farther under the vehicle. Some exhaust systems add Sensor 3, and some platforms use sensor technology or PID scaling that does not resemble a conventional narrowband voltage graph. Hybrid and start-stop operation can also delay monitor completion.

Inline Engine

Normally only Bank 1 exists, but the correct downstream sensor still requires an exhaust diagram.

V or Flat Engine

Bank 1 follows cylinder numbering, not a universal driver or passenger side.

Special Monitor

Fuel-cut timing, sensor type and pass/fail thresholds remain OEM- and calibration-specific.

OEM examples are not universal fixes. Honda documented a software branch for selected 2011–2012 Insight vehicles, and other manufacturers have issued narrowly defined P0139 calibration updates. Confirm the exact VIN and calibration before programming or replacing parts.
Reasonable DIY Checks
  • Save codes, freeze frame and readiness.
  • Confirm Bank 1, Sensor 2 by VIN.
  • Inspect cool, accessible wiring and exhaust joints.
  • Graph supported O2 and fuel-trim data safely.
Professional-Level Work
  • Low-pressure exhaust leak testing.
  • Heater load, voltage-drop and waveform tests.
  • Seized sensor, thread or catalyst repair.
  • PCM programming or catalyst diagnosis.

Seven Verified iCARZONE O2 Sensor Guides

These live guides separate B1S2 circuit low/high, no activity, heater faults, upstream slow response and the corresponding Bank 2 Sensor 2 slow-response code.

P0136
B1S2 O2 Sensor Circuit Malfunction
P0137
B1S2 O2 Sensor Circuit Low
P0138
B1S2 O2 Sensor Circuit High
P0140
B1S2 O2 Sensor No Activity
P0141
B1S2 O2 Sensor Heater Circuit
P0133
B1S1 O2 Sensor Slow Response
P0159
B2S2 O2 Sensor Slow Response

For the broader index, use the iCARZONE OBD-II fault-code center.

Frequently Asked Questions About P0139

What does P0139 mean?
P0139 means the PCM judged the Bank 1 Sensor 2 oxygen-sensor circuit response too slow during a defined monitor. It does not by itself prove that the downstream sensor or catalytic converter failed.
Where is Bank 1 Sensor 2?
Bank 1 is the side containing cylinder 1; an inline engine normally has only Bank 1. Sensor 2 is the first downstream sensor after that bank’s catalyst. Confirm unusual multi-catalyst layouts by VIN.
Is P0139 the upstream oxygen sensor?
No. P0139 points to Bank 1 Sensor 2, normally the downstream or post-catalyst sensor. P0133 is the corresponding general slow-response code for Bank 1 Sensor 1.
Does P0139 mean the catalytic converter is bad?
No. Exhaust leaks, heater or wiring faults, sensor contamination, mixture problems and calibration can all affect the monitor. Diagnose P0420 and catalyst evidence separately.
Can UR1000 diagnose P0139?
On supported vehicles, UR1000 can save the full scan and freeze frame, display available B1S1/B1S2 and fuel-trim data, check readiness and preserve reports. Exact PIDs vary by VIN and ECU.
Can I drive with P0139?
A stable vehicle with only a steady MIL can usually be driven cautiously for prompt diagnosis. Stop for a flashing MIL, severe misfire, major power loss, catalyst overheating, smoke or strong fuel or exhaust odor.
Should B1S2 switch rapidly between 0.1 and 0.9 volts?
Not as a universal rule. A functioning catalyst normally smooths downstream oxygen changes, and some vehicles use different sensor technology or PID scaling. Follow the exact OEM monitor and specification.
How do I clear P0139 safely?
Save the original evidence, repair the proven cause, then clear eligible codes. Complete the correct drive cycle and confirm monitor readiness, current/pending status and companion codes before declaring success.

Sources and Technical Review Notes

Bottom line: P0139 reports a slow Bank 1 downstream oxygen-sensor response. Prove the monitored exhaust change, sealing, heater, wiring and actual sensor response before replacing B1S2, the catalyst or the PCM.
iC
Reviewed by iCARZONE Tech Team

VIN-specific sensor locations, monitor conditions, circuit diagrams, exhaust safety procedures and current OEM service information take priority over this general guide.

Oxygen sensors and catalysts can cause severe burns, running engines produce carbon monoxide, and under-vehicle work requires approved lifting and support equipment. This article is educational and is not a substitute for professional diagnosis.