What P0139 means
02 Sensor Circuit Slow Response (Bank 1 Sensor 2)
How to diagnose
- Read freeze frame data. When P0139 set, your ECU snapshotted every sensor at that moment. This tells you what the car was doing when the fault triggered — RPM, load, coolant temp, throttle position, fuel trims. Almost always enough info to figure out the cause without further testing.
- Check for related codes. If you have P0139 plus other codes, fix the earliest one in the chain first — a single root cause often triggers multiple codes.
- Cheap fixes first. For most codes, the order is: inspect connectors + wiring (free), check for vacuum leaks (cheap), replace sensor (moderate), replace mechanical part (expensive). Follow that ladder before dropping money on parts.
Manufacturer-specific descriptions
Different manufacturers use P0139 for slightly different failure modes:
| BMW | O2 Sensor Circuit Slow Response |
| FORD | Heated O2 Sensor Circuit Slow Response (Bank 1, Sensor 2) |
| HONDA | Secondary Heated Oxygen Sensor (Secondary HO2S) Slow Response |
| MITSUBISHI | O2 Sensor Circuit Slow Response (Bank 1 Sensor 2); Oxygen sensor(rear) response |
| NISSAN | O2 Sensor Circuit Slow Response (Bank 1 Sensor 2) |
| VOLKSWAGEN | HO2S Slow Response Bank 1 Sensor 2 |
Read P0139 on your own car
DIY Fix My Car reads P0139 plus 25,000+ other DTC codes with manufacturer-specific descriptions. Includes freeze frame data, likely fix parts, and AI mechanic chat. Download free.