An oxygen sensor fault usually needs a check soon because it can affect fuel use and emissions.
Trouble codes · P0133
P0133 meaning
O2 Sensor Circuit Slow Response Bank 1 Sensor 1
The same first verdict Car Assist shows from this code alone, before a diagnosis. The code does not tell the app whether the warning light is flashing.
What this code doesn’t tell you
A code says where the car noticed a problem. It does not prove which part failed, whether the fault is still present, or what repair this car needs. The engine, other codes, freeze frame and live readings matter.
Usual causes, most likely first
The order is a starting point across cars; the exact engine and its readings can change it.
- Ageing or contaminated upstream oxygen sensor
- Exhaust leak ahead of the sensor
- Mixture fault or poor sensor connection
What you can check
Record whether fuel use or idle quality changed and keep the freeze frame. A workshop can compare sensor response with fuel trims before replacing anything.
Words the car uses
- Stored codes
- Faults the car has confirmed and saved. They help explain a check-engine light, but a code alone does not prove which part needs replacing.
- Freeze frame
- A snapshot of the engine's readings when a fault was recorded. It helps show the conditions that caused the fault, even if the car now seems fine.
- Readiness monitors
- The car's own checks of its emissions systems. An unfinished check means the car has not tested that system yet, so a clean code scan may miss a fault.
- Oxygen sensor (O2)
- A sensor that measures oxygen in the exhaust. It helps control fuel use or check the catalytic converter; B1/B2 name cylinder groups and S1/S2 name sensor positions.
- Fuel trims (STFT and LTFT)
- How much the engine computer adds to or takes away from its planned fuel amount. STFT is the immediate adjustment and LTFT is the learned adjustment; B1 and B2 identify groups of cylinders, and large adjustments can point to an air or fuel problem.
Find out what’s causing yours
Car Assist reads your car through an OBD-II adapter and your iPhone. Its assistant uses the codes, freeze frame and live readings to work through the likely cause with you.