Fault codeP0133

MODERATE

O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)

What this code means

O2 Sensor Circuit Slow Response (Bank 1 Sensor 1).

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

“Sensor 1” is the oxygen sensor upstream of the catalytic converter, the one that regulates the mixture; “Sensor 2” sits downstream and monitors the converter.

P0133 is a generic code: it means the same thing on every make. P0133 belongs to family P01xx — Fuel and air metering. Its page explains how to read a code and groups all its codes by part.

Oxygen sensor (lambda sensor)

The oxygen sensor is screwed into the exhaust and measures how much oxygen is left after combustion. The upstream sensor (sensor 1) lets the control unit keep the mixture right on the ideal ratio, correcting it many times a second; the downstream sensor (sensor 2), after the catalytic converter, checks that the converter is doing its job.

Symptoms

Few symptoms beyond the light and slightly higher consumption; sometimes a failed emissions test. The upstream sensor still swings, but too slowly: the control unit times how long it takes to switch between rich and lean, and finds it above the threshold.

Common causes

Oxygen sensor simply worn out — the normal outcome past 120,000 to 160,000 km
Sensor contaminated by oil, silicone or coolant
Exhaust leak near the sensor
Weak sensor heater, not bringing it to working temperature
Damaged loom or connector

How to diagnose it

Watch the sensor in live data with the engine warm: fewer than one swing per second at steady idle, or rounded edges, mark a tired sensor. Check there is no audible exhaust leak upstream, nor oil consumption that could have poisoned it.

On a high-mileage car with its original sensor, replacement is rarely a mistake.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

O2 sensor voltage, bank 1 sensor 1
O2 sensor voltage, bank 1 sensor 2
Short term fuel trim bank 1
Long term fuel trim bank 1
Commanded equivalence ratio (lambda)

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →