GarageGuideOxygen sensor (bank 1, sensor 1)
P0133 Code: O2 Sensor Slow Response (Bank 1 Sensor 1)
P0133 stands for "O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)." The upstream oxygen sensor sits in the exhaust before the catalytic converter and tells the computer whether the engine is running rich or lean. A healthy sensor switches back and forth quickly.
When the sensor reacts too slowly, the computer cannot fine-tune the fuel mixture well and sets P0133. "Bank 1" is the side of the engine with cylinder 1 (four-cylinder engines only have bank 1), and "sensor 1" is the one before the catalytic converter.
P0133 is not an emergency. Left alone for a long time, a poorly controlled fuel mixture can waste gas and eventually harm the catalytic converter.
You can keep driving normally while you plan the repair. Get it fixed before an emissions test and to protect the catalytic converter.
Symptoms you might notice
- Check engine light on
- Slightly worse gas mileage
- Usually no change in how the car drives
- Sometimes a rough or uneven idle
- Failed emissions inspection
Common causes
Aging or contaminated O2 sensor
The most common cause. Oxygen sensors slow down with age, often around 100,000 miles, and contamination from oil or coolant burning in the engine speeds that up.
Exhaust leak before the sensor
A leak at the exhaust manifold or a cracked pipe lets outside air in and confuses the sensor's readings.
Wiring or connector problem
A damaged, melted or corroded wire or connector can slow or distort the sensor's signal.
Vacuum leak or fuel problem
An air leak or fuel delivery issue can make the mixture harder to control, which can show up as a slow sensor.
Failed sensor heater
If the sensor's built-in heater is weak, the sensor stays cool and responds slowly, sometimes alongside code P0135.
Before you buy parts
Rule out an exhaust leak and damaged wiring before buying a sensor. If the sensor is original on a car with over 100,000 miles, replacing it is a reasonable first step.
What to check
- Read all codes. Other codes, like misfires or lean codes, can point to the real cause.
- Look at the wiring to the upstream O2 sensor for damage, melting or rubbing on the exhaust.
- Listen for an exhaust leak, a ticking or puffing sound near the engine at cold start.
- Check the sensor's age. If it is original on a high-mileage car, it is the likely culprit.
- Make sure the engine is not burning oil or coolant, which can contaminate a new sensor.
Possible repairs
Replace the upstream O2 sensor
The most common fix. With an O2 sensor socket and penetrating oil, it is often a beginner-to-intermediate DIY job. Buy the correct sensor for bank 1 sensor 1.
Repair an exhaust leak
Fixing a leaking manifold gasket or cracked pipe ahead of the sensor.
Repair wiring or the connector
Splicing a damaged wire or replacing a corroded connector.
Fix the underlying engine issue
If the engine is burning oil or coolant, fix that first or the new sensor may fail again.
Parts and tools that may help
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Frequently asked questions
What does P0133 mean?
The upstream oxygen sensor on bank 1 is responding too slowly to changes in the exhaust, so the computer cannot control the fuel mixture as precisely.
Can I drive with P0133?
Yes, usually without any change in how the car drives. Fix it soon to protect gas mileage and the catalytic converter.
Where is bank 1 sensor 1?
Sensor 1 is the oxygen sensor before the catalytic converter. Bank 1 is the side of the engine with cylinder 1; on four-cylinder engines there is only one bank.
Will a new O2 sensor fix P0133?
Usually, if the old sensor is aged and there is no exhaust leak or wiring damage. Check those first so you do not replace a good sensor.