O2 Sensor Slow Response (Bank 1 Sensor 2)
The P0139 code indicates that the oxygen sensor (Bank 1, Sensor 2) is responding too slowly to changes in exhaust oxygen content. This sensor is located downstream of the catalytic converter and monitors its efficiency. A slow response can be caused by a failing sensor, exhaust leaks, or contamination from oil or coolant. This can lead to reduced fuel economy and increased emissions.
1. Most Common Causes
- Faulty oxygen sensor (Bank 1 Sensor 2)
- Exhaust leak near the sensor
- Contaminated sensor (oil, coolant, or fuel additives)
- Wiring or connector issues
2. Symptoms You Will Notice
- Check Engine Light on
- Poor fuel economy
- Failed emissions test
- Possible rough idle or hesitation
3. Step-by-Step Diagnosis
Step 1: Scan the vehicle for codes and record freeze frame data.
Step 2: Visually inspect the O2 sensor wiring and connector for damage.
Step 3: Check for exhaust leaks upstream of the sensor.
Step 4: Use a scan tool to monitor O2 sensor voltage and response time.
Step 5: If sensor response is slow, replace the oxygen sensor.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but it may cause poor fuel economy and increased emissions. It's best to fix it soon.
How much does it cost to fix?▼
Typically $50-$150 for the sensor plus labor if you don't DIY.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.