What this code means

The PCM detected that the upstream O2 sensor on Bank 1 has an abnormal rich-to-lean vs lean-to-rich transition time ratio. The time the sensor takes to switch in one direction is significantly different from the other, indicating sensor degradation.

Symptoms

• Check Engine Light
• Poor fuel economy
• Rough running
• Failed emissions test
• Subtle performance loss

Common causes

• Degraded or aging O2 sensor
• Exhaust leak near sensor
• Fuel system imbalance
• Contaminated O2 sensor (coolant or oil)

Diagnostic steps

1. Monitor O2 sensor waveform for asymmetric transitions
2. Check for exhaust leaks
3. Test fuel pressure and trim values
4. Inspect sensor for contamination

Repair notes

Replace upstream O2 sensor Bank 1. Often appears alongside P1133.

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