P0139
O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)P0139 is a generic OBD-II powertrain diagnostic trouble code: O2 Sensor Circuit Slow Response (Bank 1 Sensor 2). It is logged by the engine control unit when the o2/lambda monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0139 means
P0139 is set when the ECM determines that the downstream oxygen sensor (Bank 1 Sensor 2) is responding too slowly to changes in exhaust gas composition. Unlike P0137 and P0138 which flag voltage level faults, P0139 is a response-rate fault: the sensor element is producing a signal, but it transitions between voltage states more slowly than the manufacturer's calibrated threshold—typically defined as a switching time greater than approximately 1.2 seconds between the lean and rich thresholds during a specific test cycle.
A sluggish downstream sensor almost always indicates an aged or contaminated sensor element. The zirconia or titania ceramic that generates the electrochemical voltage becomes less reactive over time as it is exposed to high temperatures and exhaust contaminants such as silicone (from coolant leaks or RTV sealant), phosphorus (from oil consumption), and sulphur (from fuel additives). These contaminants coat the sensor's porous electrode surface and slow the diffusion of oxygen ions through the electrolyte, resulting in a waveform that is damped relative to a healthy sensor.
Because the downstream sensor is used to measure catalytic converter efficiency by comparing its switching rate and amplitude against the upstream sensor, a slow Sensor 2 response can produce a false P0420 (catalyst efficiency below threshold) code—the ECM cannot distinguish between a slow sensor and a slow catalyst. If P0420 and P0139 are both present, resolving P0139 first is essential before drawing any conclusions about catalyst condition.
Common causes
Most-frequently reported root causes when P0139 is logged.
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1
Aged or worn sensor element with degraded electrode porosity (most common cause by mileage)
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2
Silicone contamination from coolant ingestion or improper use of silicone RTV sealant near the intake
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3
Phosphorus/lead contamination from excessive oil consumption coating the sensor tip
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4
Sulphur poisoning from high-sulphur fuel deposits on the sensor element
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5
Sensor heater circuit weakness causing element to run below optimal temperature and reducing response speed
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6
Exhaust leak near the sensor biasing the baseline and reducing effective swing amplitude
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7
Degraded catalytic converter producing a slowly oscillating exhaust that the sensor reflects faithfully (sensor is not at fault)
Symptoms drivers notice
How to diagnose P0139
A typical diagnostic flow when this code is present.
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1
Note whether P0420 is also stored; if so, do not condemn the catalytic converter until P0139 is resolved and the catalyst monitor has re-run with a new sensor installed
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2
Check for signs of oil consumption (blue smoke, high oil usage) or coolant loss (white exhaust, sweet smell) that could indicate sensor contamination rather than simple age-related wear
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3
With engine fully warm, observe live HO2S B1S2 waveform on a scan tool with graphing capability; a healthy sensor should respond to upstream oscillations within approximately 300 ms—a slow, rounded waveform with transitions exceeding 1–2 seconds confirms sluggish response
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4
Compare upstream (B1S1) and downstream (B1S2) waveforms side by side; if B1S1 is oscillating normally and B1S2 is flat or very slow, the downstream sensor is the fault; if both sensors are slow, suspect a fuel system or vacuum issue
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5
Inspect the sensor heater circuit: measure heater resistance at the sensor pins (4–30 Ω typical) and verify the heater supply fuse is intact; a weak heater keeps the element below optimal temperature
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6
Check the sensor tip visually after removal: white glassy deposits indicate silicone poisoning; black sooty deposits suggest oil or fuel contamination; a grey or clean tip is consistent with normal end-of-life wear
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7
Replace the downstream O2 sensor; if P0420 also clears after a full catalyst monitor drive cycle, the sensor was the root cause; if P0420 persists, proceed to catalytic converter efficiency testing
Related powertrain codes
- P0040 — Upstream Oxygen Sensors Swapped From Bank To Bank
- P0041 — Downstream Oxygen Sensors Swapped From Bank To Bank
- P0130 — O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor I)
- P0132 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
- P0133 — O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
Frequently asked questions
How can P0139 cause a false P0420 catalyst fault?
The ECM measures catalyst efficiency by comparing the switching frequency and amplitude of the downstream sensor against the upstream sensor. If the downstream sensor responds slowly, the ECM interprets this damped waveform as evidence that the catalyst is not absorbing oxygen fluctuations efficiently—identical to what a degraded catalyst would produce. Replacing the sensor often clears both P0139 and the associated P0420.
Is P0139 a sign of imminent sensor failure?
Yes, slow response is a precursor to complete sensor failure. The contamination or age-related degradation that causes sluggish switching will eventually progress to a fixed or absent signal. Replacing the sensor at the P0139 stage is more straightforward than diagnosing a completely dead sensor later.
Can fuel additives cause P0139?
High-sulphur fuels or cheap fuel additives containing silicone carriers can deposit contaminants on the sensor tip and cause sluggish response. Switching to a quality top-tier fuel will not reverse existing contamination but can slow future degradation after sensor replacement.
Can the sensor be cleaned rather than replaced?
There are commercial sensor cleaning products, but their effectiveness on a contaminated zirconia element is limited and unreliable. Given the relatively low cost of a downstream sensor compared to a catalytic converter, replacement is the recommended repair whenever P0139 is confirmed.
Disabling P0139 in software
RaceTune can permanently disable P0139 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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