Oxygen sensors explained

Oxygen sensors, also called lambda sensors, measure how much oxygen is left in the exhaust. The engine computer uses them to keep the mixture right and to check the catalytic converter.

Updated 16 October 2026

What they measure

Lambda is the ratio between the air the engine gets and the air it needs to burn its fuel exactly. At lambda 1, about 14.7 parts of air to 1 of petrol by weight, there’s just enough air. Below 1 the mixture is rich (too much fuel), above 1 it’s lean (too much air).

A petrol engine’s catalytic converter cleans the exhaust best close to lambda 1. The sensor doesn’t measure the air or fuel going in, but the oxygen left in the exhaust, and from that the engine computer corrects the fuel it injects. That correction is the fuel trim.

Sensor 1, sensor 2, bank 1, bank 2

Sensor 1 (upstream): In front of the catalytic converter. The engine computer steers the mixture by it.
Sensor 2 (downstream): Behind the catalytic converter. Mainly used to check that the converter still works.
Bank 1: The side of the engine with cylinder 1. An engine with its cylinders in one row has only bank 1.
Bank 2: The other side of a V or flat engine, often with its own converter and sensors.

So “bank 1, sensor 1” is the front sensor on the side with cylinder 1. Where that is on your engine, the service information for your car shows.

Narrowband and wide-band sensors

Narrowband (switching): Gives a voltage that switches between roughly 0.1 V (lean) and 0.9 V (rich), around 0.45 V at lambda 1. On a warm petrol engine the front one keeps switching back and forth, about once a second or faster at idle, and faster at higher engine speed.
Wide-band: Measures how rich or lean the mixture is over a wide range, so it doesn’t switch. Many newer petrol cars have one in front of the converter and a switching sensor behind it. Some diesels use wide-band sensors too.

Don’t judge a wide-band sensor by the switching numbers of a narrowband one: it reports differently.

The heater

An oxygen sensor only works when it’s a few hundred degrees hot. A heater built into the sensor gets it there soon after a cold start. Until then the engine computer runs on stored values, without the sensor’s reading.

A heater fault means the sensor works late or not at all on short trips, so fuel use and emissions rise. The heater is part of the sensor, so a burnt-out heater usually means a new sensor; check the fuse and wiring first.

What the codes mean

A circuit code means the signal is missing, out of range or doesn’t make sense. A slow-response code means the sensor follows the mixture too slowly, often from age or contamination. P0420 isn’t a sensor code: the engine computer compares the two sensors and concludes that the converter isn’t cleaning well enough.

A sensor that reports lean or rich isn’t necessarily faulty. An exhaust leak in front of it, a vacuum leak or a fuel supply fault make it report exactly what it sees. Look at the other codes and the fuel trims before replacing it.

In live data

Read My Car’s live data shows the fuel trims, if your car reports them: how the engine computer acts on the front sensor’s reading. It doesn’t show the oxygen sensors’ own signals. As a general guide, each fuel trim stays within about ±10 % on a healthy engine.

Before replacing a sensor

Check the wiring and connector near the exhaust for heat damage, and the fuse for the heater.
Look for exhaust leaks in front of the sensor.
Ask why it failed if it’s contaminated: oil or coolant burning in the engine, or unsuitable sealant, can coat a new sensor too.
Fit the type your engine needs. Narrowband and wide-band sensors aren’t interchangeable.

Questions

Which one is bank 1, sensor 1?

The sensor in front of the catalytic converter on the side of the engine with cylinder 1. On an engine with its cylinders in one row, it’s simply the front sensor.

Can I drive with an oxygen sensor fault?

Usually for a while: most cars drive normally, but fuel use and emissions can rise. Have it checked soon, and sooner if the engine runs badly. If the check engine light flashes, stop when it’s safe to do so.

Should I replace the sensors for P0420?

Not straight away. Fix any oxygen sensor, misfire or mixture codes first, and check for exhaust leaks. P0420 itself usually means the converter.