OBD-II trouble code
P0139: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)
The downstream oxygen sensor on bank 1 still works, but it reacts too slowly. The PCM ran a response-time test, forced a change in exhaust mixture, and the sensor took longer than allowed to follow it — the classic signature of a 'lazy', aging sensor.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $450
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0139 mean?
P0139 is different in character from the other downstream-sensor codes. P0137 and P0138 are about the signal being stuck at the wrong voltage. P0140 is about no signal at all. P0139 is about timing: the sensor produces a voltage and it moves in the right direction, but it gets there too slowly. The code is a response-time result. It is not a mileage verdict.
Here's how the PCM catches it. Periodically the computer runs an oxygen-sensor response test, deliberately nudging the air-fuel mixture and timing how long the downstream sensor takes to react. A fresh sensor switches crisply. As the platinum sensing element ages, gets coated with combustion deposits, or simply wears out, its response slows — the voltage trace gets lazy and rounded instead of sharp. When the measured switch time exceeds the threshold, P0139 sets.
A slow result is not sorted by age. Heat cycling, combustion deposits, silicone, and coolant can slow the element, and a heater that leaves it cool will do the same. Check the heater, the connector, contamination, and an exhaust leak, then replace the sensor when the response monitor still fails. Follow this vehicle's maintenance schedule rather than a mileage. Wiring stays on the list because a high-resistance connection can round the same trace off. P0139 carries a low severity rating: the car drives normally and the main consequence is a blocked emissions readiness monitor.
Common causes
- Downstream sensor that still fails the response monitor after the heater, the connector, and contamination have been checked
- Carbon or combustion-deposit buildup on the sensing element slowing its reaction
- Silicone contamination from the wrong RTV sealant used in a prior repair
- Coolant contamination from a small internal leak
- Weak or failing sensor heater letting the sensor run too cool to respond quickly
- Oil-fouled or corroded connector adding resistance and dulling the signal
- An aging catalytic converter changing the downstream exhaust profile the sensor reads
- Minor exhaust leak near the sensor disturbing the response pattern
Symptoms
- Check Engine Light on, with no change in how the car drives
- Failed emissions test or a catalyst monitor that won't set to 'ready'
- Very slight loss of fuel economy in some cases
- Often no change in power, idle, or starting, because this sensor is not the fuel-control sensor
- Sometimes appears alongside a catalyst-efficiency code (P0420) as the sensor and cat both age together
Diagnostic steps
- 1.Pull freeze-frame data to see the conditions when the code set — P0139 is set by a specific response test, so the freeze frame helps confirm it's a genuine slow-response result.
- 2.Do not try to judge this from a normal live-data graph. Behind a catalyst that is storing oxygen, the downstream trace is steadier than the upstream trace. No resting voltage window is stated for every post-catalyst sensor. A crisp trace that mirrors the upstream sensor is a catalyst finding, not proof the sensor is fast. A flat trace confirms nothing about response time.
- 3.Judge response from the PCM's own test instead. The code is set by an intrusive monitor that deliberately changes the mixture and times the sensor, so read that monitor's result and limit in Mode 06 where your scan tool exposes it, or watch the sensor during a commanded rich/lean excursion. A generic scan tool's refresh rate is too slow and too variable to measure a switch time off a live graph, so the monitor's own number is worth far more than anything you can eyeball.
- 4.Do not sort the cause by mileage. A slow result on the monitor is the finding. Check the heater, the connector, contamination, and an exhaust leak whether the sensor is old or new. Replace it when the response test still fails. Follow this vehicle's maintenance schedule rather than a universal mileage.
- 5.Inspect the connector for oil, corrosion, or moisture that could add resistance and dull the signal.
- 6.Confirm the heater is working — a sensor running cooler than designed will respond slowly even if the element is otherwise fine.
- 7.Look for silicone or coolant contamination on any sensor that fails the response test, including a sensor that was recently installed.
- 8.If the heater, the connector, contamination, and the exhaust leak are clear and the monitor still reports slow, replace the downstream sensor.
Repair cost
$150 – $450
The low end of the printed bracket is a connector or a heater circuit that is keeping the element cool. The high end is the downstream sensor after the response monitor still fails. Read the bracket as a planning span, not a quoted price. Price the connector, the heater repair, or the sensor from the labor time in the service information. A slow trace is not assumed to be wear.
Estimate your repair
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Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.