P0139Powertrain
Powertrain · SAE
O2 Sensor Bank 1 Sensor 2 – Slow Response
Meaning
The downstream oxygen sensor on Bank 1 exhibits an excessively slow response time. In a healthy system the downstream sensor should remain relatively stable; abnormal switching or slow response indicates catalyst degradation or sensor failure.
Common causes
- Degraded catalytic converter with reduced oxygen storage capacity
- Aged or contaminated downstream O2 sensor (slow response)
- Sensor poisoning from lead, silicon, or sulphur
- Intermittent wiring with elevated resistance in the signal circuit
- Exhaust leak near the sensor distorting the reading
Symptoms
- MIL (Check Engine) warning light on
- Possible failure of emissions test
- No noticeable driveability symptom in most cases
- Slightly increased fuel consumption in some operating conditions
- Unreliable catalyst monitoring by the ECU
Severity
Lieve: The vehicle remains fully driveable; the fault primarily indicates catalyst or sensor degradation with an impact on emissions.
What to do
- Read DTCs and analyse the downstream sensor waveform using a scan tool capable of graphing live data.
- Check catalyst efficiency by comparing upstream and downstream signals: sensor B1S2 should show very little switching.
- Visually inspect sensor B1S2 for signs of contamination (white silicon deposits, reddish lead deposits).
- Measure the sensor heater resistance and voltage response at operating temperature.
- Replace the catalytic converter if efficiency is insufficient; replace the downstream O2 sensor B1S2 if the sensor is the faulty component.