P0133 — Oxygen sensor circuit slow response — bank 1, sensor 1

P0133 indicates a signal response slower than the monitor expects for bank 1, sensor 1. This is the upstream sensor used for mixture control. Slow response differs from complete absence of activity.

Affected system: Powertrain

Symptoms

• Engine warning lamp and a stored code.
• Higher fuel use or uneven running if mixture control is disturbed.

Possible causes

• Aged or deposit-contaminated sensing element.
• Insufficient heating, exhaust leaks or mixture faults affecting the response test.

What to check

• Confirm B1S1 in the engine diagram and save related codes and freeze-frame data before clearing anything.
• With the exhaust cold, inspect the B1S1 harness for melted insulation, rubbing, water in the connector and loose terminals.
• Once warm, record the response to mixture changes using the OEM test; assess reaction time rather than the average reading alone.
• Check heating and fuel trims; identify the sensor type because a wideband sensor signal requires a different interpretation.
• After correcting the confirmed fault, repeat the OEM monitor conditions and check for code recurrence, beyond merely extinguishing the lamp.

Also written as: P0133-00, P013300, PO133 (some scan tools add the fault-type byte, e.g. -00).