P0139: O2 Sensor Circuit Slow Response (Bank 1, Sensor 2)

P0139 means the downstream oxygen sensor on Bank 1 (post-catalyst, Sensor 2) is not switching between rich and lean readings quickly enough during the ECU's catalyst test. The downstream sensor should stay relatively flat under normal driving but must respond predictably during diagnostic sweeps. Slow response usually points to an aged sensor or a contaminated element.

P0139 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0139 and illuminates the check engine light.

If your vehicle has triggered P0139, you may notice one or more of these symptoms:

• Check engine light illuminated
• Slight reduction in fuel economy
• Possible failed emissions test
• No noticeable driveability symptoms
• May appear with P0420 if the converter is also fading

Here are the most common causes of P0139, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Safe to drive short term. A slow downstream sensor distorts the converter monitoring strategy, so a developing catalytic converter problem may be masked. Plan a repair within a few weeks to keep your emissions diagnostics working properly.

Follow these steps to pinpoint the root cause of P0139:

1. Read freeze frame data and look for companion codes (P0420, P0136) that indicate a converter or sensor cluster issue
2. Inspect the exhaust system between the catalytic converter and downstream sensor for leaks that could distort the gas sample
3. Inspect the Bank 1 Sensor 2 connector and wiring for corrosion, heat damage, or partial open circuits
4. Use a scan tool to graph downstream O2 voltage during a steady cruise and during deceleration fuel cut. A healthy sensor reacts within roughly 100 ms to the lean spike when fuel is cut
5. Substitute a known-good sensor. Sensor response time is the only definitive test, so if the new sensor responds correctly, the original was aged out

Repair costs for P0139 vary depending on the root cause and your vehicle. Here are typical estimates:

P0139 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0139 or indicate related issues: