The rear oxygen sensor reads steadier than the front one, but it still has to react quickly when the exhaust changes. If it lags behind, the engine computer cannot trust its view of the catalytic converter. A P0139 code means the bank 1 downstream oxygen sensor is responding too slowly.
This guide explains how the engine computer tests the rear sensor, what slows it down and how to fix it.
P0139 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 39 | Specific fault | O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) |
What the P0139 Code Means
The SAE J2012 standard defines the P0139 code as O2 Sensor Circuit Slow Response (Bank 1 Sensor 2). Sensor 2 is the oxygen sensor after the catalytic converter on the side of the engine with cylinder 1.
Many powertrain control modules (PCMs) test the rear sensor during deceleration fuel cutoff. When you lift off the gas at speed, the injectors shut off, and the exhaust fills with oxygen. A healthy rear sensor should drop from its normal reading of about 0.6 to 0.8 volt to below about 0.2 volt quickly.
The PCM times that drop. If the sensor takes too long, the code sets. Some makers also time the rise back to rich when fuel returns.
Symptoms of a Slow Rear O2 Sensor
Most drivers notice only the warning light. Signs of a P0139 code include:
- Check engine light on
- Catalyst monitor that will not complete
- Failed emissions test
- Small drop in fuel economy on some engines
What Causes the P0139 Code?
Age causes many cases, but other problems can slow the sensor. These are the usual causes:
- Aging oxygen sensor. The element slows down after many miles of heat.
- Contaminated sensor. Oil, coolant or silicone deposits slow its reaction.
- Exhaust leak near the sensor. Outside air confuses the readings.
- Weak heater. A sensor that runs cool responds slowly.
- Corroded connector or wiring. High resistance dulls the signal.
- Leaking injector. Fuel dripping during fuel cutoff keeps the exhaust from going fully lean.
How to Diagnose the P0139 Code
You need a scan tool that can graph live data. A safe road where you can coast from about 40 mph helps too.
Graph the Coasting Drop
Warm the engine fully. Graph the bank 1 sensor 2 voltage while you accelerate to about 40 mph, then let off the gas and coast in gear. A healthy sensor should fall below about 0.2 volt within a few seconds. A slow, sloping drop points to a lazy sensor.
Do a Propane Test
At idle, add a small amount of propane near the intake to drive the mixture rich. The rear sensor should rise steadily toward 0.8 volt or more. Then pull a vacuum hose to make the mixture lean, and it should fall. Compare its speed with the front sensor on the same bank.
Rule Out Leaks and Heater Problems
Inspect the exhaust around the rear sensor for rust holes and soot marks. Measure heater resistance at the sensor connector and compare it with the spec. If the heater checks out and there are no leaks, the sensor is most likely worn.
How to Fix the P0139 Code
Repair the cause, then clear the code and drive normally, with some coasting from highway speeds.
- Replace the downstream oxygen sensor if it fails the coasting test.
- Fix exhaust leaks near the sensor first.
- Repair the heater circuit if it tests open or weak.
- Fix the contamination source, such as oil burning or a coolant leak.
- Clean the connector if the pins are corroded.
Before you buy a sensor, check the freeze frame data saved with the code. It shows the speed and engine load when the test failed. If it set during a long coast from highway speed, the PCM had a clean chance to watch the sensor, which makes a worn sensor the most likely cause.
Repair Cost Breakdown
A new sensor fixes most P0139 code cases. This table shows typical costs.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Downstream O2 sensor replacement | $30 to $120 | $120 to $350 |
| Exhaust leak repair | $20 to $100 | $150 to $500 |
| Connector cleaning | $10 to $15 | $50 to $100 |
| Diagnostic time | Free with your own tools | $100 to $200 |
Is It Safe to Drive with a P0139 Code?
Yes, in most cases. The front sensor still controls fuel, and the engine runs normally.
The main concern is that the PCM cannot check the catalytic converter properly, so you may fail an emissions test. Replace the sensor when you can, and keep an eye out for other codes.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0134 | O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1) |
| P0135 | O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1) |
| P0136 | O2 Sensor Circuit Malfunction (Bank 1 Sensor 2) |
| P0137 | O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) |
| P0138 | O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) |
Restoring a Quick Rear O2 Sensor Response
A P0139 code means the bank 1 downstream oxygen sensor reacts too slowly. Age, contamination and exhaust leaks cause most cases.
- Graph the sensor drop during coasting.
- Do a propane test to compare it with the front sensor.
- Check for exhaust leaks and heater faults.
- Replace the sensor if it stays slow.
Frequently Asked Questions
What Is Deceleration Fuel Cutoff?
When you lift off the gas at speed, many engines shut off the injectors completely to save fuel. The exhaust then fills with oxygen, and the oxygen sensors should read very lean. The PCM uses this moment to test how fast the rear sensor responds.
Can a Leaking Injector Cause the P0139 Code?
It can. If an injector drips fuel during fuel cutoff, the exhaust never gets fully lean. The rear sensor then drops slowly or not far enough, and the PCM blames the sensor. Look for negative fuel trims and a fuel smell to spot this problem.
Should I Replace the Front Sensor Too?
Not unless it shows its own fault. The front sensor on bank 1 has separate codes, such as P0133 for slow response. Graph it at 2,000 rpm. If it switches quickly, leave it. If it is old and slow, replacing both at once can save labor.
Why Does My Catalyst Monitor Not Finish?
The PCM needs a working rear sensor to judge the catalytic converter. While a rear sensor code is stored, the catalyst monitor usually will not run. Fix the sensor fault first, then drive normally for several days so the monitor can complete.
How Long Does a Rear O2 Sensor Last?
Rear sensors often last longer than front sensors, since the catalytic converter shields them from some contaminants. Many reach 100,000 miles or more. Oil burning, coolant leaks and rich running shorten their life. A slow drop during coasting is a common sign of wear.