P2196Powertrain
Powertrain · SAE
O2 Sensor Signal Stuck Rich — Bank 1, Sensor 1
Meaning
The upstream lambda sensor on bank 1 persistently reports a rich mixture signal without the typical oscillations of closed-loop control. The ECU cannot correct towards stoichiometry despite applying maximum negative fuel trim.
Common causes
- Faulty or contaminated O2 sensor B1S1 (signal stuck rich, > 0.7 V)
- Leaking or over-fuelling injectors on bank 1
- Excessive fuel pressure (faulty regulator)
- Faulty MAF sensor reporting insufficient airflow
- EVAP system fault causing excessive purge fuel vapour
- Common rail fuel pressure relief valve leaking (diesel context)
Symptoms
- Check engine light (MIL) on
- Bank 1 long-term fuel trim (LTFT) strongly negative
- Raw fuel smell from exhaust
- Increased fuel consumption
- Possible black smoke (petrol engine over-rich condition)
Severity
Moderate to high: a chronically rich mixture damages the catalyst with unburned fuel and can cause oil dilution.
What to do
- Read bank 1 STFT/LTFT fuel trims to confirm rich tendency
- Check fuel pressure and injector condition (static leak-down test with engine off)
- Monitor O2 sensor B1S1 signal in real time: if it does not oscillate, sensor is faulty
- Check EVAP system for abnormal purge valve opening
- If O2 signal is fixed high, replace O2 sensor B1S1