P0133 — O2 Sensor Circuit Slow Response (Bank 1, Sensor 1)
Low severityWhat it means
P0133 is stored when the engine control unit sees a fault involving the upstream oxygen sensor on bank 1 (sensor 1). The bank 1 upstream oxygen sensor measures exhaust oxygen before the catalytic converter and is the main input the control unit uses to trim fuel. “Slow response” means the oxygen sensor still works but switches between rich and lean too slowly — a classic sign of an aged sensor.
Common symptoms
- Slightly higher fuel use or a failed emissions test
- Rough running or higher fuel use
- Check engine light
- Failed emissions test
Likely causes
- Aged or contaminated oxygen sensor
- Silicone or oil contamination on the sensor tip
- Aged or contaminated oxygen sensor
- An exhaust leak near the sensor
What to check first
- Watch the sensor's switching speed in live data and compare with a healthy sensor
- Watch the upstream sensor switch between rich and lean in live data
- Inspect for exhaust leaks and check the sensor wiring
Can you keep driving?
Drivable short-term; a fuel-control sensor fault wastes fuel and can affect the catalytic converter, so repair soon.
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