Emissions code
P0139: Downstream oxygen sensor slow to respond (bank 1)
How this range is built
| Part of the job | Low | High |
|---|---|---|
| Parts | $40 | $180 |
| Labor: 0.6 to 1 hours × $125 to $175 an hour | $75 | $175 |
| Fees: shop supplies, disposal, diagnostics | $0 | $15 |
| Total, rounded | $110 | $370 |
The hourly rate is the BLS Occupational Employment and Wage Statistics, May 2025 average pay for this trade in Boston, $31.54 an hour, times 4 to 5.5 to cover what a shop bills (overhead, tools, insurance, profit). Parts are a band from budget aftermarket to the manufacturer part. Full method.
Not a repair shop, estimates only. Typical range, estimate. Not a quote. Your car, parts and shop change the number.
This is the range for the most common fix, oxygen sensor replacement (one sensor). Which repair your car needs depends on a proper diagnosis.
The oxygen sensor behind the catalytic converter on cylinder 1's bank reacts too slowly when the exhaust mix changes. Its job is to confirm the converter is working, and a sluggish one cannot do that well. Age, heat and oil or coolant contamination are the usual reasons.
- Safe to drive?
- Usually yes, for now
- Most common cause
- Aging O2 sensor
Safe to drive. It mostly affects emissions testing.
What you may notice
- Check-engine light on
- No change in how the car drives
- May fail the emissions readiness check
Common causes, most likely first
- Aging O2 sensor
- Exhaust leak
- Contamination from oil or coolant burning
- Wiring issue
Check this first
Ask whether an exhaust leak has been ruled out ahead of the sensor, since leaks distort the readings and can mimic a slow sensor. Also mention if the engine burns oil, which can coat the sensor.
Other repairs that sometimes fix P0139
| Repair | Typical range, Boston |
|---|---|
| Catalytic converter replacement | $400 to $1,900 |
| Diagnostic scan (check engine light) | $60 to $290 |
Before you pay
- Ask the shop what test showed this part is the cause, not just that the code appeared.
- Ask for parts and labor listed separately, and for the hours.
- Ask for a written OK before any extra work. Massachusetts rules require written authorization before repairs.
Got a quote already? Check it against the range. Want to see if your car has a recall tied to this problem? Check recalls by VIN.
Need to read a code yourself? OBD-II scanners on eBay (affiliate link, disclosure).
P0139 questions
Why is it slow?
Sensors wear with heat cycles, and soot, oil and other contaminants slowly coat the tip until it lags. The change is gradual, so you rarely notice it until the computer flags it.
Will it hurt mileage?
Not much. The sensor behind the converter mostly checks the converter, and it has little say over fuel delivery, so your mileage should stay about the same.
When do O2 sensors wear out?
Often somewhere between 80,000 and 120,000 miles, though it depends on the engine and driving. Sensors that fail earlier can point to oil burning or a coolant leak that is damaging them.
How much does it cost to fix P0139?
The most common fix, oxygen sensor replacement (one sensor), has a typical range of $110 to $370 in Boston. It is an estimate built from parts prices and local labor wages, not a quote. Your car and shop change it.
Related codes
- P0136: Downstream oxygen sensor circuit problem (bank 1)
- P0420: Catalytic converter not working well (bank 1)
- P0141: Oxygen sensor heater fault (bank 1, after converter)
More emissions codes
Code meaning written in our own words. Some codes mean different things on different makes, so check your scan tool or manual. Not a repair shop, estimates only.